ammo is cheap and many inexpensive military surplus gun partscould be used to ease the build.

To keep fabrication relatively easy, I decided to make thereceiver from high-quality 4130 tubing. Fire control parts requireconsiderable engineering, so I decided that AR-15 fire controlparts, which are well-designed, easy to understand, inexpensiveand widely available, would be the way to go.

much machining, it would discourage potential builders. A sim-ple single-shot firearm would be the easiest project for the hob-byist, but interest would probably be minimal. Semi-autos are themost popular today, so that’s what I decided to design and make.

I decided to use the simplest operating system, straight blow-back. This dictated the use of a low powered pistol cartridge suchas .380 ACP, 9mm Luger or .45 ACP. I chose 9mm Luger because

At some point every hobbyist gunsmith wonders if he couldbuild his own gun. Many advance into this segment oftheir hobby by “building” a gun from a parts kit. Kits like

the AR-15 are very easy to build with factory-made parts. Theskilled hobbyist gunsmith can assemble a box of AR-15 partsinto a functional firearm in a couple hours or less.

One may however question if this is building or just assem-bling, which does not require much gunsmithing skill or knowl-edge. Someone completely unfamiliar with firearms could do it,which is exactly the point with modern military firearms. Theless skilled labor required, the better. A lot of those in our most-ly male firearms fraternity would be surprised to know that gunsare mainly assembled in the factory setting by women.

Someone else has done all the engineering work. They havedecided what parts to make and how to make them, what materi-al will be used to make them, and how all the parts interact toperform the intended function. The other extreme of “building”your own gun would be to design and fabricate all the parts need-ed to create a functioning firearm. This extreme is not reallypractical for the hobbyist. The amount of design work, technicalknowledge, and tooling to create every single part would be costand labor prohibitive.

Even custom gun makers use many parts manufactured byothers to keep manufacturing cost reasonable. For the hobbyistgunsmith the “build it yourself ” option should fall somewherebetween these two extremes.

A couple years ago I decided to design and build a gun proj-ect for an SGN article. I created a design and built and docu-mented the project but other article projects always seemed topush this project to the back burner. I have done more than 20articles for SGN over the last two or three years and my articleidea list was getting pretty low, so I figured I better get this proj-ect out of mothballs.

I wanted to use some factory-made parts to keep the projectdoable for the hobbyist. If it was too difficult or required too

Selecting Parts,Making the Bolt

Part 1

Have you wanted to build a gun that’s truly your own? Here Matthews shows how to takesome steel and a handful of parts and turn it intoa gun. It takes the right tools and techniques, butif you’ve followed his other stories, you can do it.

A lot of Matthews’ previous projects could largely be accomplished with hand tools,but this one is a bit more advanced and requires some pieces be made on a lathe.

Ever want to make your own gunfrom the ground up? The SGN-9requires some manufactured parts,but offers plenty of opportunity to show your gunsmithing skill.

Ever want to make your own gunfrom the ground up? The SGN-9requires some manufactured parts,but offers plenty of opportunity to show your gunsmithing skill.

Building

SGN-9theBuilding

SGN-9the

BY STEVEN MATTHEWS

Every semi-auto firearm requires a bolt, and these can be verydifficult for the hobbyist to make from scratch. What I neededwas an inexpensive surplus bolt that could be the “raw material”for making a semi-automatic bolt. A barrel for this project couldbe made from a commercially available 9mm barrel blank butthese could be expensive ($50-100).

A less expensive option is to use a military surplus barrel.Most military surplus 9mm barrels are from submachine gunsand are under the BATFE mandated 16-inch overall length need-ed for a semi-automatic rifle. These barrels can, however, bemodified by permanently installing an extension to bring themup to the required length.

I wanted magazines that were very inexpensive and were sin-gle-feed. Double-feed magazines are more reliable, but posesome design difficulties for the home builder. A design that is farfrom perfect in the theoretical sense, but that was perfect for thisproject, is the Sten magazine.

Sten magazines can be found for less than $10 each, I havegotten them for as little as $3 each on occasion. What I ended upwith was a design that used many factory-made parts but wouldcontain enough hand made parts to be far beyond an “assemblyjob” gun project.

Since this project was going to be the basis of a SHOTGUNNEWS article, I decided to call it the SGN-9 Semi-AutoCarbine. In this project the builder would have to make thereceiver, trigger housing, magazine housing, barrel shroud,stock, sight mount, and various small parts.

The bolt will be “made” by heavily modifying an SMG bolt to semi-auto configuration. Purchased parts will include thebolt, barrel, fire control group, firing pin, grip, recoil spring,recoil pad, and various small parts and hardware. All these partswill have to be hand fitted together into a functional and safefirearm.

Before we get into the specifics of this project several issuesneed addressing. This article is not going to be a step-by-stepbuild article. I will give some general details of how to make andfit the parts but the builder is going to be totally responsible for

the fit, function, and safety of allparts.

Given the variables associated withhome-made parts, considerable handfitting and modifying of parts may beneeded to ensure proper functioning.A minor size variation of one part mayresult in some other part not function-ing correctly. This project is advancedgunsmithing and the builder mustknow how each and every part is madeand how it functions.

If you do not completely under-stand how this firearm functions, donot build it! The builder is the gun-smith/manufacturer on this projectand has the final say on what is cor-rect and safe. Neither SHOTGUNNEWS nor the author are responsi-ble for the construction, use andsafety of this firearm.

It is also the builder’s responsibil-ity to ensure that this firearm is legalto build and possess in his location.Although this firearm is designed asa legal semi-automatic rifle, somelocations may restrict ownership ofcertain types of firearms. I have no

professional firearms engineering experience and this project ispresented solely as an amateur’s attempt at firearms design.

Parts Selection

The first step is to obtain the needed parts. When I decided tostart this project, the first thing I did was to look through my pileof gun parts to see if there was anything I could utilize. I foundthat I had many parts that could be used.

I had a full (minus receiver) parts set for a Soumi M31 sub-machine gun, designed and built in Finland in the 1930s (see 1/10and 1/20/08 issues). The barrel was 3/4" in diameter and was 12inches long, which would work very well and meet BATFEguidelines when a 41⁄2-inch extension was permanently installed.

The original full-auto bolt featured a fixed firing pin andwould be prohibited under BATFE regulations, since the agencyconsiders a fixed firing pin a machine gun feature. The fixed fir-ing pin is, however, removable and the bolt can easily be modi-fied to use a conventional firing pin, making it a semi-automat-ic system.

The Suomi kit also contained a magazine release and springthat could be used. The best part of using Suomi M31 parts iscost. Suomi M31 parts sets are advertised in SGN for as little as$60. This is a great price for a high quality barrel, bolt and mag-azine lever.

The AR-15 hammer, trigger, safety, disconnector, and associ-ated pins and springs would be used for the fire control system.AR-15 parts are widely available from SGN advertisers. I gotmine from Brownells, since I was going to obtain finishing sup-plies from them as well. You might associate Brownells solelywith gunsmithing supplies, but they are an excellent source forgun parts, both standard and custom.

You could order parts individually, but I found that theyoffered a complete DPMS AR-15 lower receiver parts set for theprice of just a few individual parts. The unused parts can besaved for future projects. Their prices were also competitive withother AR-15 parts suppliers.

I ordered an AR-15 firing pin and standard length recoilspring, along with a 9mm Luger chambering reamer to adjustchamber depth and headspace after assembly. This reamer mayor may not be needed depending on how precisely you assembleyour project.

Combining Soumi M31 and AR-15 parts with some steeltubing and common steel stock gives you the raw mate-rials for a 9mm carbine. You’ll have to supply the labor.

The Soumi bolt is heat treated, and Matthews founddrills and cutters barely scratched it. Careful annealingis required to make it soft enough for easy cutting.

The bolt will need to be reduced to 1.00-inch outside diameter to fit inside thenew tubular receiver, and there’s really no substitute for a lathe for doing that.

Although it’s reduced in diameter, the bolt needs to retainthe original outside contour, with a reduced center sec-tion between two larger ends. This will reduce binding.

A milling machine is sure a lot easier, but Matthewsexperimented and found that the large slot in the bolt canbe made by drilling multiple holes and filing out the rest.

The 7.16" slot in the bolt allows conversion to an AR-15hammer and other fire control components. Matthewspicked AR-15 parts because they are cheap and available.

BUILDING THE SGN-9 | PART 1

The receiver of the SGN-9 will be made from 11⁄4"x.120" wallseamless 4130 chrome-moly tubing. This material is availablefrom aircraft supply dealers. I recommend about 4-5 feet of thisjust in case you make a mistake and need to make a new piece.A barrel shroud will be made from 11⁄4"x.062" chrome-moly tub-ing. A couple feet will be plenty.

I chose this thinner size to keep weight down, but you coulduse the heavier receiver tubing if you want, you will just have toadjust your barrel shroud bushing size to compensate for the sizedifference. A foot of 7/8"x.120" wall chrome-moly tubing willmake a barrel extension.

The trigger housing/lower receiver and magazine housing willbe made from common 1"x 2"x3/32" rectangular steel tube. Thisis available at well equipped steel dealers. A couple feet will bemore than enough.

A foot or two of solid 11⁄4" solid chrome-moly rod will be usedto make the barrel bushing, bolt stop, and end cap. This is alsoavailable from aircraft suppliers.

A common grade 53⁄4"x6" bolt can be used to fabricate the boltextension that will be attached to the rear of the bolt.

A foot each of 3/4"x3/4" and 1"x1" square steel stock will beused to make some small parts.

The buttplate will be made from a piece of 1/4"x2" flat steelstock. Get a couple feet in case of a foul-up.

A 2-foot piece of 3/16"x1/2" flat stock will be needed for thestock construction if you choose to make a skeltonized buttstock.

I wanted a rubber buttplate and I used a buttplate and spacerfrom an FN/FAL rifle. These are available for a few bucks fromgun parts dealers. If you don’t mind a solid steel buttplate, youcan delete this item.

Various small pieces of steel, pins, and screws will be usedand discussed in the fabrication text. Total parts cost for thisproject excluding finishing supplies will be about $175, but this

figure can be reduced considerably if youare a frugal shopper and search out bar-gains.

Tools Required

Parts are readily available for this proj-ect but nothing can be done unless youhave the right tools. You’ll need basichome workshop tools such as a vise,files, drills, taps hammers, saws, punch-es, etc. The more tools you have, the bet-ter; more tools usually make the job eas-ier.

A metal turning lathe is absolutelyneeded for this project since many of theparts are turned to precise sizes. Thelathe work is not very difficult and youdon’t have to be a master machinist, butyou just can’t do this project without alathe and the skill needed to operate it.

A milling machine will greatly help onthis project but is not an absolute neces-sity. Many of the milling machine madeparts can be made by hand with saws,files, drills and grinders. It just takes longer and requires moreskill to make them by hand.

I made gun parts by hand for more than 20 years before Icould afford a milling machine. Hand making parts is slow andtedious, but it does work and you don’t have to spend hundredson a mill. Hand making parts will really help you hone your fab-rication skills, too.

A drill press with a vise is just about a must-have tool for thisjob since most (the author included) can’t drill accurately andsquarely with a hand drill. A Dremel Moto -Tool or small airangle grinder will really help keep hand filling to a minimum.

One power tool that I find especially useful in parts fabricat-ing is a bench type disc sander. This tool is great for shaping steelthat needs to be flat without dips and gouges.

Of course, you need accurate measuring tools such as a scaleor ruler graduated in 32nds and 64ths. A set of dial calipers isneeded for making measurements more precise than can be donewith a ruler. One other must-have tool is an arc welder, sincemany of the parts are welded in place. I recommend a MIGwelder since, they are better for small fabrication projects.

A common stick welder can be used, but you must use verysmall electrodes and use welding methods that control heat dis-tortion since stick welders generally generate much more heatand distortion in the workpiece than a MIG welder.

Bolt Fabrication/Modification

The first part that needs to be made is the bolt. This is thehardest part of the project and everything else depends on gettingthe bolt right. If you can’t get the bolt fabricated correctly, thereis no use doing the rest of the project. Making the semi-auto-

matic bolt is the absolute heart of thisproject.

The “raw material” for this part isthe Suomi M31 submachine gun bolt.Modifying this fixed firing pin bolt toconventional firing pin operation ismuch easier than fabricating a newbolt from scratch.

The bolt will be modified byremoving the fixed firing pin andthen “bushing” the hole, machiningthe interior of the bolt to accept anAR-15 firing pin, and cutting a ham-mer slot in the bolt to allow use of thesemi-automatic AR-15 hammer andother fire control parts.

The bolt will also be reduced indiameter so that it will fit the newSGN-9 receiver. A bolt handle will beadded to the new semi-automatic boltbut it will be done later since its loca-tion will be affected by the location ofother parts not yet made.

A bolt extension will be added tothe rear of the bolt to increase itsmass since so much weight wasremoved when it was modified for the

new receiver. Firearms operating by the blowback principle musthave bolts that have enough mass to keep the bolt closed untilchamber pressures have dropped to safe levels. If blowback boltsare too light, functioning and safety problems will arise.

To begin the bolt work, you need to remove the extractor byusing a small screwdriver to pry up at the front of the bolt faceand then slide it forward and off. Before doing any machiningwork, you may have to soften the bolt by annealing it with heat.It may be too hard to machine easily.

This bolt is a high quality item and was heat treated for max-imum wear resistance. The bolt I used in this project was so hardthat drills and lathe cutters would barely cut the steel. To annealthe bolt, use a propane or oxy-acetylene torch to heat the bolt tillit just begins to turn dark red. Heat it very evenly and do notoverheat it since it will make it too soft and it will wear out eas-ily.

Once you have the whole bolt a dull red color, slowly removethe heat source and allow it to cool slowly. When it’s cool, youcan check to see if you softened it enough by using a file to testhardness. If the file easily cuts the steel, it has been softened. Ifthe file doesn’t cut easily, it probably needs to be reheated a lit-tle hotter for a little longer.

The Suomi bolt features a fixed but removable firing pin. Thisfiring pin is pressed in place and then crosspinned with a small

The Soumi firing pin is pressed in place and thencrosspinned with a small pin that is about 3/4" back fromthe front of the bolt. Driving it out allows pin removal.

The former firing pin hole is tapped for a 12-28 bushing. Threading the hole mustbe done precisely; using the lathe chuck and tailstock will help keep it straight.

Any slam-fires must be avoided, both for safety and forlegal reasons. A firing pin return spring helps preventthem by keeping the firing pin from flying forward.

PART 1 | BUILDING THE SGN-9

Once the bushing is installed, a 1/16" hole needs to bedrilled in the center of the bolt face for the firing pin.Then face off the bushing flush with the bolt face.

Cont. to page 24

pin that is about 3/4" back from the front of the bolt. It is locat-ed straight up and down and sometimes is hard to see, so keeplooking if you can’t find it right away.

My bolt was so finely finished that the pin was almost invisi-ble. Once the crosspin is driven out with a small punch, the fir-ing pin can be driven out. It will come out the front of the boltand a long pin punch can be used to drive it out. This is one ofthose procedures that looks good on paper!

On my bolt, the crosspin wouldn’t budge and neither wouldthe firing pin. After bending a couple pin punches, I decided thatthe parts must have been extremely tightly pressed in place atmanufacture. I had to partially drill out both pins to relieve ten-sion before they would move. Even after annealing the bolt, thesepins were still pretty hard, so be sure to use sharp drills if youhave to drill them out.

The receiver of the SGN-9 will be made from 11⁄4"x.120" wallthickness 4130 chrome-moly tubing. This leaves an internaldiameter of 1.010". A running clearance of .010" will mean thebolt will need to be machined to 1.00". You can vary this a little,but if the bolt is too large, it will bind and if too small it will bea sloppy fit.

Place your bolt in your lathe and turn it down to this size.Profile it just like the original. The wide ends should be the spec-

ified size of 1.00" and the reduced partin the center should be .020-.030"smaller. This is especially important forsmooth functioning. If the bolt bodywas the same size all the way any dis-tortion in the receiver could cause bind-ing. After it is turned to size, chamferthe ends on both sides for smooth oper-ation when it slides in the receiver.

The next procedure will be to cut aslot in the bolt body for the AR-15 ham-mer. This slot will allow the hammer tocontact the firing pin and also allow thebolt to cock the hammer when it recip-rocates.

This 7/16" slot needs to be orientedfrom top to bottom so be sure it is locat-ed correctly before you start removingmaterial. The front will be squared upand fitted by hand, but the rear canremain rounded. The slot will extendfrom roughly .600" from the front of thebolt body and extend to within about11⁄2" from the rear of the bolt body.

The rear dimension is not too critical,but the front needs to be fairly close, because the front edge willalso stop the hammer when it strikes the firing pin. Give yourselfplenty of room here, since you will be hand fitting the bolt andhammer later.

You can mill out this slot with a milling machine but if youdon’t have a mill, it can be done with a drill press and filemethod. Draw out your slot on the bolt andthen drill several holes within the slot area andthen use files to remove the remainder of thematerial.

Hand work like this is slow but it doesn’tcost a thing! Once the slot is cut out it’s time tostart on the firing pin segment.

That nice round hole where the fixed firingpin used to be now needs to be filled back in.This procedure is a variation of the old-timeprocedure of “bushing” a worn firing pin hole.Many old-time guns wore out the firing pinholes and a new hole had to be made by drillingout the bolt face and then installing a threadedinsert and then drilling a new firing pin hole.

The hole in the front of the Suomi bolt wassized just right as a tap hole for a 12-28 tap and bolt. A 12-28 tapwas simply used to thread the hole. How deep to thread and howlong the insert should be is up to you. The longer and deeper isbetter, since what is not needed will be drilled out anyway. Ifmemory serves me correctly (it been a couple years ago) I

believe I went about 1" deep. A 12-28 Grade 5 screw was used for a

bushing. Once it was threaded in all theway, I cut it off then faced it off even withthe bolt face. Install it very tight and usepermanent thread locker before cutting itoff. A quick note here about the 12-28 tapand bolt. This size was common in yearsgone by, but has fell out of favor in mod-ern times since it is only slightly smallerthan common 1/4" tap and bolt sizes.

Bolts and taps in this size are still available, but you will haveto get them from places that carry a large selection of taps andbolts, you won’t find them at hardware stores. Brownells cata-logs this size and many large industrial suppliers also have them.

Once the insert has been installed and faced off, drill a newfiring pin hole from the front and drill the rear to accept the AR-15 firing pin. A fair amount of skill is needed for this procedure.It’s not all that hard, but it’s easy to mess up if you are not pay-ing attention to small details.

Start by chucking up the bolt in your lathe with the front of thebolt on the working side. Be absolutely sure it is centered andrunning “true.” If it’s not correct, your holes will run off-center.Also be sure your tailstock and drill chuck are running on thecenterline. If the tailstock is off more than a thousandth or two,things can get off-center also.

Start by using a very small center drill and just put a smalldimple on the bolt face for a starting point. The very tip of anAR-15 firing pin is about .060" so we will use a 1/16" (.0625")

drill to drill a hole in the bushing. Drill all the way through thebushing you installed. These small bits are very flexible, so drillwith a very light feed to keep it from walking off center.

Later holes will track off of this hole when they are drilledfrom the rear, so it has to be right. Once your hole is complete,you can turn the bolt over because all the rest of the holes will bedrilled from behind. Now you will have to drill out a profile ofthe firing pin in the center of the bolt and insert. This is whereyour gunsmithing/machining skill is going to be important.

I will give the diameters of the holes that will need to bedrilled, but depth is going to be a matter of your own judgment.You want the exact shape of an AR-15 firing pin in the bolt witha few thousandths for clearance.

Your drills will have to be very long to reach the full drillingdepths. You will have to obtain long drills or weld/solder/brazesmall bits to an extension. Drill all holes with shorter, more rigid,bits before moving on to the more flexible long bits to keep yourholes from running off center.

The tip of the firing pin is .060" and will fit through the 1/16"hole you drilled in the bolt face, but the section behind the tip isabout .100"-.102". You need to drill a hole big enough for thissection of pin and a couple thousandths for clearance. I recom-mend about .104" which is a No. 37 drill. You do not, however,drill all the way through the bushing.

This is going to be drilled in two steps, one to the approximatedepth and then it will be final drilled to the finish depth after youhave enough holes drilled to install the firing pin and hand fit itjust right. Drill with the #37 drill till the tip is about 3/32" fromthe front of the bushing. Use whatever method you like to deter-mine this drilling depth, but do not drill through the front of yourbushing.

The next wider portion of the firing pin is about 5/32" (.156").You need to drill a clearance hole for this portion of the pin. A No. 21(.157") or No. 22 (.159") drill bit is about right for this hole. This hole will be drilled to about 3/4" from the bolt face. This dimension is not critical, but try to keep it close to this depth. The large head of the firing pin is about .335" and an “R” drill (.339") may be needed for clearance in the bolt body. The existing hole in my bolt was just a little under this size and I only had to drill very little to get the head to fit in thebolt body.

To prevent slam-fires and a runaway full auto malfunction afiring pin return spring must be used. The spring will prevent thefiring pin from striking the cartridge primers when the boltreciprocates. This is a necessary feature found on most semi-automatic firearms. Do not delete this feature!

Failure to incorporate this feature could result in a malfunctionthat causes automatic fire and authorities have in the past ruledthat a malfunctioning firearm is actually an illegal full-autofirearm. You don’t want to have to convince them that your mal-function isn’t really a prohibited firearm. The penalties aresevere for unregistered full auto guns!

For a firing pin return spring I used a 7/32"x11⁄2" spring thatwould easily slide over the .156" part of the firing pin, anddrilled a recess for this spring in the bolt. Drill the recess to adepth that allows the spring to keep the pin in the rearward posi-tion. The spring should be strong enough to keep the firing pinfrom moving under recoil.

Once all your holes are drilled, you must verify that the firingpin moves freely in its recess in the bolt. A pin that binds cancause the same full auto malfunction mentioned previously, so besure it moves freely.

Extra-long tooling is required to drill through the lengthof the bolt for the firing pin. The pin and its spring are retained in the bolt body by a hex socket screw.

To compensate for the weight of material removed,Matthews added an extension to the rear of the bolt thatconveniently became the guide rod for the recoil spring.

The modified bolt (top) is far removed from the originalSoumi bolt. Changing it is a lot of work, but is certainlya lot easier than making a bolt from whole cloth.

PART 1 | BUILDING THE SGN-9

Cont. from page 22

Cont. to page 26

At this point your firing pin must be hand-fitted to your par-ticular bolt. The clearance hole for the .100" section of the firingpin now needs to be carefully drilled to allow about .035" to.040" of the firing pin tip to extend out of the bolt face. You wantas much material as possible to remain around the firing pin tipto support the rear of the cartridge. Some of your other holedepths may need to be adjusted to allow the tip to reach thisamount of extension. I used a hand drill with a long bit to do thisfinal drilling. Drill a little, then check the fit.

Once you have the firing pin fitted to the bolt, it’s time tosquare up the front of the hammer slot. The goal here is to havethe square edge of the slot flush with the head of the firing pinwhen it is all the way forward. This is to ensure that when thehammer strikes the firing pin and drives it forward and pushes itto its full extension (.035"-.040") that the hammer comes to reston the bolt and not the firing pin.

You want the hammer to “bottom out” on the bolt, not the fir-ing pin, reducing stress on the latter. Place your firing pin in thebolt without the spring in place and note how much material needsto be removed from the front of the slot to make it flush with thepin head. Hand-file the rounded slot to a square shape. Checkyour progress often and be sure you do not remove too muchmaterial. Remember you can always take off more to get it to theright size but you can’t put it back on if you take off too much.

After your firing pin is fitted and moves smoothly, a firing pinretainer set screw need to be installed in the bolt. I used a 8-32set screw. This set screw will function just like the cotter key typeof retainer used on an AR-15 bolt carrier. Locate it so it allowsfull travel of the pin but doesn’t allow the pin to extend too farout of the rear of the bolt.

The screw needs to extend into the recess in the firing pinhead but not to deep to contact the pin’s shank. Locating the setscrew so that the pin extends about .100" rearward of the edge ofthe squared-out slot is about right. When you tap this hole, tap ita little shallow so that the last few threads are very tight; this willhelp keep the screw tight. A little removable thread locker shouldalso be used.

Adapting the firing pin to this bolt does require a lot of handfitting to get right. I highly recommend that you proceed withgreat caution when drilling all these holes and shaping the bolt.Due to variations on your hand-made and -fitted parts (plus themanufacturing tolerances in the bolt and firing pin) your actualdimensions may be different than the ones I used. Since one lit-tle mistake can ruin the bolt, be absolutely sure of anything youdo. Remember, part of gunsmithing is making decisions con-cerning how to do any operation the correct way.

Since I’d reduced the diameter of the SGN-9 bolt for the newsize receiver and machined a slot in it, a lot of weight was lostand needed to be replaced to maintain an acceptable bolt weight.I added an extension to the rear of the bolt that brought boltweight back to within a couple ounces of the original.

This extension needs to be about 3/4" in diameter and extendout of the back of the bolt by 4 inches. It also needs to extendinto the interior of the bolt till it is even with the hammer slot. Iused a common 3/4" Grade 5 bolt for this part. You may drill outthe rear of the bolt and tap it for this extension or you maymachine the extension to fit the existing hole in the rear of thebolt .and then press fit the stub into the rear of the bolt.

The existing hole in the rear of the bolt is rather small and anodd size so you may want to drill it to a standard size such as5/8", and then fit your extension to that size. Weather you threadthe hole or use the press-fit method drill a hole for a crosspin tokeep it from working loose.

Roll or spring pins work well for this. After this extension isinstalled I recommend chucking the bolt up in a lathe and truingup the extension so it is exactly in line with the bolt body. If youinstalled it half ways straight removing .020"-.030" will probablybe enough.

The hammer must slide easily over the bottom of the bolt. Ashallow flat groove the same width of the hammer slot needs tobe cut in the bottom. This groove has to be cut to a depth that isonly slightly higher than the diameter of the front section of thebolt. It has to be shaped so that the hammer will slide over easi-ly on both the forward and rearward strokes.

A generously sized radius will be formed on the back of thegroove, but the front will need a more gradual taper so the ham-

mer will slide back over easily. The hammer will be cocked whenthe bolt moves to the rear and the hammer will have to slide backover this part when it moves forward.

This shaping to provide smooth operation is going to be total-ly dependent on the shape of the modified AR-15 hammer thatwill be made later. The location of your lower receiver and firecontrol group will also affect the operation of these parts.Considerable hand fitting and shaping of these two surfaces willbe required to get a smooth operating bolt and hammer. Youmight as well wait till you have these parts made before you cutthis groove. Just don’t forget it later.

Similarly, you will need to make a bolt handle and fit it to thebolt. You’ll drill a two-diameter hole in the bolt and the handlewill interlock with the bolt and help align the bolt in the receiv-er. This hole cannot be located until the receiver is made.

If you have made it this far on this project without major prob-lems, the rest of the project will be much easier. I can’t overem-phasize the importance of fabricating the bolt correctly. All othergun functions depend on the bolt working properly. It may seemhard to make, but if you take your time and think things through,it’s not all that difficult.

I spent about six to eight hours working on the bolt, but thatincluded all the design work. Your time may be more or less, itjust depends on how skilled you are. I’m no genius; if I could doit so can you!

PART 1 | BUILDING THE SGN-9

SGN-9 Raw Materials and Parts Sources

AR-15 Parts, Accessories, and GunsmithingSupplies — Brownells, 200 S. Front St., Montezuma,Iowa, 50171,1-800-741-0015, brownells.com

4130 tubing and round stock — Wicks Aircraft Supply,410 Pine St., Highland, Ill., 62249, 1-800-221-9425,wicksaircraft.com

Suomi M31 Parts Sets — Cope’s Distributing, P.O. Box 70, Pitsburg, Ohio, 45358, 1-937-901-9072,copesdist.com

Cont. from page 24