JOURNAL OF THE OPTICAL SOCIETY OF AMERICAV

An Optical Sighting Gauge for the Garand Rifle*

C. B. SITTERSON, JR., AND NORMAN F. BARNESGeneral Engineering Laboratory, General Electric CGonpany, Schenectady, New York

THE American soldier is traditionally anexcellent marksman. Part of the founda-

tion of this tradition is due to the everydayfamiliarity of the average American male withfirearms, and a great part is due to the excellentquality of our military rifles.

Regardless, however, of the ability of themarksman or the quality of the rifle, no personcan shoot accurately unless he is sure of the cor-rect position at which to set his sight withrespect to the distance between himself and histarget. In other words, at a distance of 100yards a particular sight setting is necessary foraccurate shooting. At any other distance adifferent sight setting is necessary. Also, thesight must be set laterally in order to compen-sate for the drift of the bullet caused by crosswinds. The latter adjustment is referred to as"windage." The adjustment for various distancesis called "elevation."

Military firearms use ammunition which isstandardized. Within close limits, therefore, allbullets leave the rifle with the same velocity andfollow the same trajectory. Based upon thistrajectory, it is possible to compute the elevationand windage calibrations for the rifles. The rearsight of the Garand rifle is calibrated in thismanner.

It then remains to set one point of the calibra-tion correctly with respect to the index on therifle. When one point is indexed, all others willbe correct. This operation is known as "zero-ing,"or "sighting-in" a rifle, and it is for this operationthat the optical rifle sighting gauge was de-veloped. The zero windage and the 100-yardelevation are the values which are indexed.

When rifles are manufactured in small quanti-ties, the sighting operation is done by the trialand error method of actually firing the rifles onan indoor 100-yard range. This involves theexpenditure of thousands of dollars for theconstruction of the range; the services of twooperators-one to fire the rifles, and one to

* Paper presented at the 'lventy-Eighth Annual Meetingof the Optical Society of America, Pittsburgh, Pennsyl-vania, October 7-9, 1943.

attend to the targets 100 yards away; and thefiring of many rounds of ammunition. Althoughthis method is unimpeachably accurate, it isslow and expensive. For instance, two men onone range can sight only 60 or 70 rifles in 8hours, using from 8 to 13 rounds of ammunitionper rifle.

When rifles are manufactured in quantitiessuch as are involved in our present militaryproduction, the expense, as well as the necessarilyincreased equipment and time, of the abovemethod of sighting prohibits its use. Many in-door ranges, dozens of men, and thousands ofrounds of ammunition would be necessary. Somefaster, less expensive method is needed.

The authorities of Springfield Armory, lookingahead to their problems, realized this need andcame to the General Electric Company with theproblem and with the idea that optics probablyoffered a solution to it. Accordingly, the problemwas referred to the General Engineering Labo-ratory.

Many methods of optically or mechanicallysighting rifles had been tried previously. Thisprior art had gained for itself the general titleof "bore-sighting," because in its use the sightshad been aligned with respect to the entirelength of the axis of the bore of the rifle. Previoussuccess had not been encouraging.

It is probable that if the construction of theGarand rifle were such that we could have con-veniently worked upon the entire length of thebore, we would have done so in the initial phaseof our development. However, we could notconveniently do this. Some other method ofattack was necessary.

The idea then occurred to N. F. Barnes and1K. R. Geiser that inasmuch as every rifle barrelhas minor bends and twists, the last few inchesof the bore probably do the major part ofdetermining he trajectory of the bullet. If thishypothesis were true, it would mean that thesights should be aligned with respect to the lastfew inches of the bore, rather than with respectto the entire length of the bore.

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OPTICAL SIGHTING GAUGE

Lamp Switch-

ElevatingHand whe J

FIG. 1. Optical rifle sighting gauge, oblique front view.

Consultation with the Experimental Depart-ment of Springfield Armory strengthened ourbelief in the truth of this hypothesis. Ir. JohnGarand stated that he felt sure it was true andthat we would have to determine experimentallythe optimum length of bore to use. Accordingly,it was decided to conduct experiments in orderto determine definitely the feasibility of the idea.

This investigation was conducted at theSpringfield Armory with the cooperation of theArmory personnel, and covered a period ofseveral months. Tests utilizing the last to 8inches of the muzzle end of the bore for thedetermination of the projectile trajectory showedthat the last 4 inches substantially determinethe path of the bullet for a given set of firingconditions. A subsequent correlation of the dataobtained indicated that it would be possible toalign the sights with respect to these last 4 inchesof the bore with such accuracy that, at a distanceof 100 yards, approximately 82 percent of thecenters of impact of a group of five shots firedfrom each rifle would fall within 3 inches of thecenter of the target. Upon the basis of theseresults it was decided to design and manufactureequipment incorporating the tested principle.

A further consideration of the problem made

it apparent that the sighting equipment musthave as its function the transferring of the sightsetting of a so-called standard rifle to any otherrifle. This standard rifle has been correctlysighted by firing. In order to accomplish thisresult the sighting gauge establishes two lines inspace, one corresponding to the bore direction asdetermined by the standard rifle and the othercorresponding to the line of sight of the standardrifle. Basically, the gauge, as shown in Fig. 1,consists of three parts:

1. A fixture for rigidly holding the rifle andaccurately positioning it without placing anystrain on the barrel.

2. A target optical system which indicates thealignment in space of the bore as determined bythe last 4 inches.

3. Two optical projectors for producing mag-nified images of the front and rear sights andfor indicating the sight positions relative to therifle bore.

The muzzle of the rifle is gripped by a spring-operated 3-jaw chuck, and the rifle receiver isgripped by a clamp, the curved bottom plate ofthe trigger assembly being seated in a V-groovein the base of the clamp. Positioning of the rifleis accomplished by elevating and traversing the

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C. B. SITTERSON, JR., AND N. F. BARNES

rear of the rifle by means of two hand wheels,the left hand wheel controlling the elevation,and the right one the traverse.

When the rear of the rifle is traversed andelevated, the resulting motion takes place abouta point at the center of the bore at the muzzle,this point also being the center of the chuck.The small gimbal which supports the chuck isfor the purpose of preventing any strain beingplaced upon the barrel.

The alignment of the bore axis in space is de-termined by a fixed point-the bore axis at themuzzle-and two angles measured in a horizontaland vertical plane and indicated by an externaltarget optical system. This system consists of alight source and field lens, a cross-shaped aper-ture, a concave mirror mounted upon the endof a 4-inch bore plug which is inserted into themuzzle end of the rifle, an adjustable planemirror above the light source, and a mirror andground glass screen on the main fixture.

The light bulb, lens, aperture, and adjustablemirror are mounted in a separate target unit,shown in Fig. 2, approximately 2 meters infront of the main fixture and facing it.

Light leaves the lamp bulb, goes through thelens and aperture, and falls upon the concavemirror which is mounted upon the end of thebore plug, with its axis coincident with that of

FIG. 2. Target unit for optical rifle sighting gauge.

the plug, as shown in Fig. 3. From the concavemirror the light is reflected to the adjustableplane mirror mounted above the light source,and then back again toward the main fixture tothe mirror which throws the cross image uponthe viewing screen. The concave mirror has afocal length such that it focuses an image ofthe cross aperture upon the viewing screen.A cross is marked upon this screen to indicatewhere the reflected cross should fall, the twocrosses being superimposed when the rifle iscorrectly positioned.

The sight projectors are optical systems de-signed to produce magnified images of the sightsupon the viewing screens. The resulting magnifi-cation is about 25 times. The correct positionsof the sights relative to the bore axis for correctfiring at a 100-yard range are indicated byoutlined sight image positions on the viewingscreens. Also shown on these screens are "one-click" variations, each so-called "click" repre-senting a variation in position of either sight inelevation or windage necessary to move thecenter of impact on the target 1 inch at 100yards.

The projectors are mounted on separate armsso that they may be moved up to permit theinsertion and removal of the rifle. The two armsrotate about a common shaft, and are raised bya handle attached to the shaft. The arms andprojectors are held in the up position by anautomatic latch. Each projector arm has a dashpot which gently lowers the projector over thesight, thereby preventing jarring when the pro-jectors are dropped. Individual stops determinethe downward positions.

The sighting equipment is first adjusted to astandard rifle. This rifle is one which has beenselected by actual targeting on the basis of cor-rect grouping and shooting at 100 yards' rangewith the sights set at their theoretically correctpositions. This standard is clamped in the fix-ture, the sight projectors lowered, and the boreplug inserted into the muzzle end of the.bore.

With the elevation and traversing motions ofthe fixture centered, the sight projector opticsare so adjusted that the two sight images fallupon their respective outlined positions. Finallythe plane mirror on the target unit is adjustedso that the image of the cross aperture, as

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formed by the concave mirror on the bore plug,is superimposed upon the marked cross on themiddle viewing screen.

After these adjustments have been made, thesighting equipment is ready for the productionalignment of rifle sights. In order to carry outsuch an alignment, a rifle is clamped into positionin the sighting gauge, and the bore plug carryingthe concave mirror is inserted into the muzzleend of the bore. The elevation and the traverseof the fixture are then adjusted so that the crossimage on the center viewing screen is superim-posed upon the marked cross on that screen.The rifle is then in the correct position to fire atthe same target point as the standard rifle withwhich the gauge was adjusted. The projectors arethen lowered into position so that the magnifiedsight images appear upon the viewing screens,thereby showing the sight locations relative tothose positions required for correct alignment.The sights are then moved to their correctpositions and indexed.

These sighting operations, which require aspace of less than one-twentieth of that requiredby a 100-yard range, can be performed at a rateof about 40 per hour. Correlation of the sightinggauge data with actual firing data shows thatapproximately 85 percent of the centers of im-pact of a group of five shots fired from each

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FIG. 3. Bore plug and mirror assembly. Cover removed toshow adjustment screws.

rifle fall within 3 inches of the target center ata distance of 100 yards. Rifle variations withinmanufacturing tolerances account for deviationsbetween optical sighting alignment and that ob-tained from firing. Those manufacturing varia-tions which would cause a marked differencebetween the center of impact and the targetsighting point cannot all be detected by theoptical sighting gauge since the operation of thelatter assumes that mechanically perfect riflesare used.

It is hoped that the principles embodied in thisequipment will be applicable to the "sighting-in"of other armament including that for aircraft sothat the saving of time, equipment, and moneythrough the use of the sighting gauge for theGarand rifle can be extended to the rest of thearmament industry.

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