The lock-time of the Sharps Borchardt action is fairly fast as it was made originally, due to the short firing-pin travel but it may be speeded-up still more, as is done with bolt-actions, by lightening the firing-pin, from the rear, to remove weight but, due to the sear notch in the pin, this hole cannot be made very large, although drilling the hole off-center enables you to make a larger hole than if the pin is centrally drilled. Some weight may also be removed from the heavy head of this firing-pin, on the top and bottom sides. If these methods do not increase lock speed enough, a new firing-pin may be made of light-weight metal to replace the steel pin.
Speeding-up lock-time on bolt-action arms is done by using stiffer springs of keystone-shaped spring-wire, by substituting alloy-steel firing-pins without cocking-knobs—as has been done at Springfield Arsenal on the "free" rifles—or by shortening firing-pin travel by grinding back the sear-notch and the face of the sear. The best results come from lightening the pin and using the stiffer spring of keystone-section wire.
Trigger-pulls can be greatly refined and lightened, thus improving accuracy, by carefully polishing the contact surfaces of triggers, hammers and sears and this is the first thing to be tried in changing a trigger-pull, before making any other alterations. Polish these contact surfaces with the hard white Arkansas stones, then examine them carefully with a high-power magnifying glass to see that the surfaces are flat and even. When reassembling them for trial, coat the surface of one or the other lightly with prussian-blue, then all points of contact can be readily seen, so that high spots can be removed or the shape altered if necessary. On bolt-actions of the Mauser type, polish the under surface of the receiver, where the rear upper portion of the trigger bears.
If the polishing process does not result in sufficient lightening of the trigger-pull, check the depth of sear engagement in the sear notch and also see if the sear nose and notch are of such shape that the hammer or firing-pin is cammed back against the main-spring pressure by the first part of the trigger-pull.
If the sear engagement seems unnecessarily deep in its notch, except in bolt-actions, drill a small hole in the back part of the notch and drive in a Vie" diameter
is pulled. At right is shown correct shape, so hammer will remain stationary as sear point is withdrawn from notch by pulling the trigger.
pin tightly, filing this off to the proper length so that it just allows the sear to engage a small amount of the notch, which will shorten its pull.
If the hammer or firing-pin is cammed back upon pulling the trigger, change the angle of the contact between the sear and notch so that upon pulling the trigger the sear moves straight forward or down out of its notch without exerting this camming action. Be careful in this operation not to bevel these surfaces in the opposite direction but keep them level, so that during the movement of the sear out of its notch there is no movement in either direction of the hammer or firing-pin, as the case may be.
These three points; polishing contact surfaces, giving the sear a shallower engagement in its notch, and straightening the contact surfaces so that no camming action occurs upon pulling the trigger, will practically always lighten the trigger-pull sufficiently. The one exception to this is when the sear-spring is too stiff, which sometimes, but very seldom occurs. If this is the case, a lighter sear spring may be fitted or the pres-
Method of pin insertion in face of sear notch in hammer to decrease depth of a notch that is too deep.
ent one may be shortened by cutting it off a little at a time, but be very careful in this operation and check and recheck time and again to see that the spring remains stiff enough to return the sear to its proper seat in its notch each time the trigger is partly pulled and then released. If the sear is not returned fully to its seat each time, a dangerous condition has been set up and the only remedy is a new sear spring.
Bolt-action trigger-pulls require a different treatment in some respects than other types. Most of the bolt actions have the double or military-type pull, to which many shooters object. In types of these actions which have an independent bolt-stop separate from the sear, this take-up or preliminary pull may be removed. One method of doing this is to place a small steel plate in the guard, on the upper side so that, its rear end prevents the trigger moving far enough forward to allow the sear to rise to its full height. This plate may be fastened in place with two small screws or two rivets. If screws are used make sure the threads fit tightly, so that they do not work loose.
A second method of eliminating the take-up pull is to drill and tap a small hole through the trigger, so that a screw passed through the trigger from rear to front
Trigger-pull adjustments for bolt actions. A shows plate riveted to guard, to take up part or all of preliminary pull. B shows same object attained by use of a small scrcw through trigger, while C shows this preliminary pull taken out by means of a pin driven tightly into a hole drilled into front of trigger so that its end bears against front edge of trigger slot in guard.
will bear against the front edge of the trigger slot in the guard to prevent the trigger moving all the way forward. Here again the threads must fit very tightly, to prevent any movement of the screw after it is adjusted.
A third method is merelv a modification of the second, in that a small hole is drilled only partly through the trigger from front to rear, then a pin is driven tightly into the hole and filed off to the proper length to limit trigger movement. This method is safer than the screw in that if this pin loosens it can only drop out, allowing the trigger to move completely forward, whereas if the screw loosens it is possible for it to screw farther forward through the trigger, so as to shorten the trigger-pull below the safety point. This however, is a remote possibility.
In lightening the trigger-pull of bolt actions, all contact surfaces are polished and trued up so that no high spots remain and the rear top edge of the sear is given a very slight radius with a fine stone, merely enough to remove the sharp edge. The face of the sear notch in the cocking piece is stoned back on a slight angle from the vertical, so that the rear face of the sear bears only for short distance at the bottom edge of this notch. If these two contact surfaces here and the contact surfaces of the two cams on top of the trigger and the under surface of the receiver where these two trigger cams bear are given a good polish, the average bolt-
Correct shape of scar and face of scar notch of bolt actions.
action trigger-pull can be brought to three pounds with no trouble. In some bolt actions however, the sear spring is unduly stiff and it will be necessary to shorten it by cutting it off, half a coil at a time and trying it. This spring should always be left stiff enough to return the sear to full engagement with the cocking piece when the trigger is given the preliminary pull and then released without completing the release of the firing-pin.
Check the rear face of the sear to make sure that it is of the proper shape so that it does not cam the firing-pin back against main-spring pressure when the trigger is pulled, but be sure that this rear face of the sear does not have so much angle that it has to force the firing-pin back to rise to its full engagement when the preliminary pull only is given and the trigger is then a
Using a small jewelers lathe to make a composition front sight head. New white or red sight beads arc easily turned to size, at high speed, in a jeweler's lathe, using either a tool rest of the type shown or a hand rest and a small chisel.
released, as this condition makes a very stiff sear spring necessary and increases the weight of the pull.
If length of the preliminary pull is not objected to, a good short, snappy, final pull can be given by grinding down the top of the rearmost of the two cams on the upper part of the trigger which bear against the bottom of the receiver. By decreasing the height of the rear cam, more work is thrown onto the front cam, which has the most leverage, and the sear is drawn farther down out of the notch by the preliminary pull before the second cam comes in contact with the bottom of the receiver to give the final pull. The increase in the length of the preliminary pull will hardly be noticeable, but a lighter trigger-pull will result.
The only work recommended on trigger-pulls of autoloading arms is a thorough polishing of all contact surfaces, as parts of these arms are usually fitted with more tolerance than other types and the action is subject to far more jar and vibration, so that if trigger-pulls are lightened too much they may soon fail to hold when a little wear appears and the arm is then unsafe.
The smooth functioning of a gun depends upon the proper fit of parts and the closer this fit is made and the better the surface of the parts, the longer the life of the action will be. To get this condition, the parts are machined or filed as closely to size while in their soft state as is safe, which is usually within .002", and are then hardened and the surfaces brought to a perfect fit with small hand stones and lapping plates. This is called hard fitting of parts.
In this work the stones used are small, fine carborundum and hard white Arkansas stones. In using such stones on small surfaces there is a tendency to use a rocking motion, so to overcome this these parts should be held in a vise or clamp with rather wide, flat topped, smooth jaws, with the part being worked upon being held in these jaws with an almost imperceptible amount above their top, so that as it is brought down to size the stone slides across the tops of the jaws, which reduces the rocking tendency to a minimum. (See Frontispiece.)
Construction of lapping plate of cast iron, for use in lapping flat parts.
A lapping plate of cast-iron is used to flat-finish parts by rubbing them over the plate, which is charged with diamond dust or other lapping compounds, by hand. The surface of this plate is crossed or checkered with small, shallow grooves and the lapping compound
Construction of cast iron lap, for lapping round holes.
being used is mixed with oil of a light body and spread upon the plate. In using these lapping-plates, do not lap a piece continually in one place but move it about all over the plate, to reduce any tendency to wear the plate unevenly.
Holes are usually lapped to size with a round lead lap, which is charged with emery and oil, although expanding cast-iron laps are sometimes used with diamond dust to lap holes. The lead lap is merely upset to increase its size by bumping the end of it lightly
from time to time, but the cast-iron lap is a sleeve with four or more slots sawed in it, half from one end to within a short distance of the opposite end and the other half sawed from the opposite end to within a short distance of the first end. These slots alternate with each other, one from one end and the next from the other end. The interior hole or bore of this sleeve is a taper. A taper-pin Teamer may be used to produce this taper and a corresponding taper is turned upon a shaft to fit the hole. Part of this shaft at the small end of the taper is turned straight and threaded for a nut smaller in diameter than the outer diameter of the sleeve and this nut forces the sleeve farther up the taper of the shaft, thus enlarging the lap as is necessary.
The sides of parts such as hammers, which must have a good finish but need not have as high a degree of fit on the sides as other parts, are polished or lapped upon a plate of steel or iron on which fine carborundum cloth is cemented. This may be cemented to the plate, either with the quick-setting wax put out for attaching grinding-paper discs to disc-grinders or water-glass may be used, although this is much harder to remove than the wax.
In choosing steel for making gun parts, the function of the part is studied to see if it is subject to impact or only wear from a sliding action. If the part is subject to impact, such as firing-pins, locking bolts or hammers, it should be made of alloy-steel or tool-steel. Locking-bolts are best made of nickel or tungsten alloy-steel, while hammers and firing-pins should be made of carbon tool-steel, although hammers can be made of manganese alloy-steel as this is very tough. Firing-pins should not be made of tool-steel of too high a carbon content, chisel or rock-drill steel being better than drill-rod for this purpose. All these steels require proper heat treatment to bring out the required hardness and toughness.
Sliding parts require a high surface hardness but usually do not need interior hardness, so these parts are usually case-hardened to resist wear and the center left soft so that in the hardening process there is no danger of cracks developing, unless the part is too thin and is given too great a chill. As case-hardening is an impregnation of the surface with carbon, a tool-steel is not of the best type to choose for a case-hardened part as the structural grain is too fine to allow good penetration of the carbon. A steel of a coarser, more open grain will answer the purpose better as it allows a deeper more even penetration of the carbon, so machinery steel, commonly called cold-rolled, is a better case-hardening steel than tool-steel. Also, due to the fact that tool-steel already contains carbon, it will develop a hardness of its own throughout during the case-hardening operation, which will defeat the soft-center idea which is one of the objects of case-hardening.
Misfires are sometimes due to incorrect steel being used for a firing-pin or may be due to incorrect hardening methods being used. It might be thought that a firing-pin of machinery steel, case-hardened, would be the best type, but the continual pounding of an action-hammer on the end of a case-hardened pin will gradually bulge the soft center of the pin, until it binds in its hole and softens the force of the firing-pin blow on the primer; or a tool-steel pin may be used and be case-hardened, which will cause cracks to appear, especially in the smaller point section of the pin which will finally break off and yet may remain in place and appear all right to a casual inspection. When the pin is in two parts, dirt will collect between the ends of the two parts or small particles of brass from primers or cartridge case will get in there and cushion the blow of the firing-pin. Using a steel of too high a carbon content may result in the same thing, too brittle a pin which will cause breakage and misfires. Firing-pins made of chiscl or rock-drill steel, then hardened and tempered in oil, will give the best service and the least trouble from misfires, as the oil hardening imparts less brittleness to the steel than does cold water and seems to develop the greatest toughness in this chisel-steel.
I have seen firing-pins made of carboloy that were ground to shape, as they were too hard to machine with a cutting tool, but they were placed in a soft steel breech-block and as the firing-pin had a long slender nose and the hole in the soft steel breech-block soon wore to the point where it was far from a close fit on the firing-pin, the pin had so much side play that, being very brittle as it was, it soon snapped off; so in this case a poor choice of steels to work together resulted in the defeat of the desired object, namely a long-life firing-pin.
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