Most of the component parts for the single-shot falling block pistol presented in this chapter are similar to the corresponding parts used in the previous semi-automatic design. Therefore, I will not go into great detail on constructing them. This being the case, do pay close attention to the drawings and dimensions as presented in the following pages.
This pistol's receiver is made from a solid steel block 3% inches long, 1V2 inches wide, and one inch thick. If it is to be chambered for a high-intensity cartridge, be sure to make the receiver from a block of quality steel. If available, S.A.E. 4340 steel is ideal for this purpose. Once machined to size and shape, the 4340 steel is heated to 1475 to 1525 degrees F., then quenched in oil, and the temper drawn at 1100 degrees F. The receiver's hardness should then equal C35 on the Rockwell scale, or a tensile strength of 160,000 pounds per square inch, and a yield strength of 138,000 pounds per square inch. So the strength of this 4340 receiver will safely handle just about any small arms cartridge commercially available today.
If only low-pressure cartridges are contemplated, then the receiver may be machined from scrap steel such as plow beam, etc. However, I strongly urge that you use a quality steel of known analysis so that it can be properly heat treated.
Cut the block of steel to the receiver dimensions previously given, by sawing, filing and grinding, ormilling.The next step is to lay out a center-line along the block's top, bottom and both ends. This line fixes the location of the barrel, breech block, hammer, and trigger, so make certain that it is centered exactly all the way around.
1 % inches back from its front edge, punch a mark on the center line at the block's top and bottom sides. These locate the center of the breech block opening, which is drilled and reamed to a finished diameter of .750 inch. One-half inch down from the block's top edge, make two
"Extractor pin 1/8"
SINGLE SHOT RECEIVER
' ! Hl/4"h punch marks on the center line, one on the front face and one on the rear. The barrel will thread into a 1/2 inch hole centered on these marks. Bore this hole completely through lengthwise. Drill and ream the length of this hole forward of the breech block opening to a finished diameter of 11/16 inch. Thread the first .800 inch of this hole back from the front face with a 3/4 inch * 16 thread. The bottom side of the breech block opening is also threaded, to a depth of 3/8 inch using a 7/8 inch x 27 thread. The portion of the breech block holeto be threaded must first be reamed out to a diameter of 27/32 inch , which is the proper tap drill diameter for a 7/8 inch x 27 thread.
I suggest holding the receiver in a lathe's four jaw chuck while it is being drilled, bored, or reamed. The threading should also be done in the lathe even if taps are used to cut the threads. "True" alignment is much more easily achieved using this method.
Cut a 1/4 inch wide slot on the center line and at the rear of the breech block opening, for the hammer and sear. If the rear end of the breech block is cut to the contour shown in the diagram before this step, the slot will be slightly easier to cut since ther will be less metal to remove. Cut away the upper portion of the 1 /2 inch lengthwise hole (to the rear of the breech block opening) until it is the same width as its diameter (.5 inch), leaving the bottom half rounded. Cut a 1/4 inch wide slot at the lower front for the upper end of the trigger, and a 1 /8 inch slot for an extractor at the left front side of the breech block opening. Some of the steel removed from these slots can be drilled out, but their inside corners will have to be removed with files and narrow chisels. The extractor slot particularly is difficult to cut to the finished shape, but with a sharp chisel and a lot of patience, it can be done.
Drill the holes for the trigger and sear pins with a No. 31 drill through both sides of the receiver. One side is drilled out to 1/8 inch. The smaller No. 31 holes will hold the 1/8 inch pins securely in place. The hole for the hammer screw is first bored with a No. 3 drill, followed by a 1/4 inch drill through one side, which is also counterbored for the screw head. Tap its opposite side for a 1 /4 inch * 28 thread. The top of the receiver forward from the breech block opening can now be rounded or filed to an octagon contour. The upper and rear end edges should be rolled or rounded off slightly, but the bottom side and front face must be kept sharp and square.
Thread a five inch long section of 7/8 inch seamless tubing with an inside diameter of 3/4 inch on one end to screw tightly into the bottom of the breech block opening. The pistol's wooden grip will be fitted around this tube.
If tubing is not available, a section of 12-gauge shotgun barrel cut from the heavier part near the breech end can be reamed almost to size and turned to the outside diameter. Then screw it in place and ream it to finished size, together with the breech block opening. This will insure correct alignment, assuring that the breech block will work as designed.
Lay out and cut the slots in the front and rear sides of this tubing as shown in the diagram, using the cutting methods described for the slide and receiver of the semiautomatic handgun. The small slot near the top is for a bar connecting the trigger and sear, and while there must be some up-and-down clearance, the sides should fit this connecting bar closely.
Construct the breech block from round stock polished to slip fit closely inside the breech block opening. Use the same 4340 steel used for the receiver if possible. It may be heat treated to the same hardness as the receiver, or left slightly harder by drawing at 1000 degrees F. This will afford a reading of C38 Rockwell, a tensile strength of 178,000 pounds per square inch, and a yield strength of 156,000 pounds per square inch for the breech block. Here again, an automobile axle or similar steel parts can be used for the breech block, but when steel of unknown analysis and composition is used, the results of the heat treatment are uncertain.
Trim the breech block to an overall length of two inches, and mark a center line completely around its top, bottom, front and rear. A 3/16 inch wide slot is cut from front to rear centered on the bottom centerline, extending upward 11/s inches. To start this slot, drill a hole slightly smaller than its finished width, through the block, then saw through
7/8 x 27 Thread
GRIP FRAME & BREECH BLOCK GUIDE
each side of the slot with a hacksaw. Square the bottom and sides of the slot by filing and chiseling.
Drill a hole as indicated from left to right for a 3/16 inch link pin. Use a slightly smaller drill for this hole first, preferably a No. 16. Then drill one side of the breech block with a 3/16 inch bit. A hardened pin from 3/16 inch drill rod should then fit tightly through these holes, serving as the link pin.
Finish the upper front face of the block to a flat surface where it fits against the breech end of the barrel. Cut it back until the flat area measures at least 5/8 inch (.625 inch) across and 11/16 inch (.6875 inch) high when measured down from the top. You must bevel this flat area's top front edge to the rear slightly, which will allow it to assist in camming the extractor shut as the breech block rises. Center and cut a rounded trough lengthwise to the top of the breech block, not more than 3/16 inch deep.
Wait until the barrel is in place and the lever and link connected before drilling the firing pin hole in the breech block. To mark its location, first turn a sharp point on a rod which is a close slip fit in the bore, and slightly longer than the barrel. With the breech block closed and the lever latched, insert the rod into the barrel, pointed end first. Once in place, tap the rod's butt with a hammer. This will accurately mark the firing pin location in the exact center of the bore, assuring that the firing pin will strike the primer in its center, and not high, low, or off to one side as many falling block designs do.
If a rimfire cartridge is intended, move the firing pin location upward the distance necessary to contact the cartridge rim.
Bore the firing pin hole with a 1/16 inch drill, followed by a 1/4 inch drill to accommodate the firing pin body and retraction spring. Also drill a 1/8 inch hole to serve as a gas vent from the top center into the firing pin hole. The vent allows gas to escape in the event of a blown primer. A firing pin retainer is also installed to hold the firing pin in place. The firing pin hole should be plugged while this hole is made with a No. 31 drill, to keep the drill from wandering. Thread the upper part of this hole with a 6 x 48 tap, and make the retainer pin to the dimensions shown.
The firing pin is made from 1/4 inch drill rod. Turn the rod's front portion until it slip fits in the firing pin hole. Actual protrusion of the pin should be .060 inch. Use a small coil spring to retract the firing pin, which occurs when the hammer is pulled back to the half-cock or safety notch.
If an extremely high intensity cartridge is contemplated, the front installed firing pin and the flat-backed breech block designs shown as alternatives in the drawings may be used for added strength and durability. This design requires a square flat face at the rear of the barrel breech block opening in the receiver.
I have built this pistol in .22 WRM and .357 magnum calibers, and used the first breech block firing pin system in both, with entirely satisfactory results. The alternate is shown simply to satisfy those who feel they must have it that way.
The barrel blank is procured or constructed in the same manner described for the other handgun. It can be about any length you want. The only requirement is that the barrel must have a 7/8 inch (.875 inch) shoulder to butt up against the forward end of the receiver. The barrel on the gun shown has an overall length of seven inches, a muzzle diameter of 5/8 inch (.625 inches), and a straight taper expanding to 7/8 inch (.875 inch) 1/2 inch forward of the thread shoulder. This leaves a cylindrical section .500 inch long and .875 inch in diameter. The barrel shank is 1.6 inches long, with the first .800 inch turned smooth to a diameter of 11/16 inch (.6875 inch), and the adjacent portion turned to3/4 inch (.750 inch), then threaded with a 3/4 inch x 16 thread.
Make sure that when the barrel is screwed in place tightly, the breech end almost contacts the face of the breech block with no more than .005 inch clearance between them.
While the barrel is in place, locate and mark its top center. A front sight made as shown is silver-soldered in place just behind the muzzle end of the barrel on the center line.
Another center line is located and marked on the bottom side of the barrel, and a nut or boss affixed to it for the forend retainer screw to thread into. Make this nut from 1/4 inch round stock, 5/16 inch long. Radius its top to fit the barrel contour, and drill and tap the nut for the size screw you have available. An 8 * 40 * 1/2 inch screw will do nicely for this, in which case the proper drill would be a No. 28. If it is necessary to use an 8 x 32 or 8 * 36 thread, both of which are available at most hardware stores (the 8 * 40 is not), then drill the hole with a No. 29 drill.
Mark the extractor location while the barrel is in place in the receiver. Remove the barrel, and cut the extractor slot with files and a narrow chisel. With this accomplished, the chamber is cut almost to depth and the barrel re-installed.
Cut the extractor from 1/8 inch sheet steel, and bend its upper end to a right angle. The extractor's upright portion should fit closely in its slot in the receiver without side play or wobble. A hole is drilled in the receiver for an extractor pivot pin with a corresponding hole through the extractor. Drill these with a No. 31 drill and tap the first part of the receiver hole with a 6 » 48 tap. Turn the threads off the lower end of a 6 * 48 screw and remove the screwhead. Cut a narrow slot across this screw's top to enable use of a screwdriver, and use it for the pivot pin.
The upper extractor leg that fits in the barrel slot is fitted by filing, spotting, and filing again until it fits. Use a round file to remove all the portion possible extending across the chamber opening. To finish cutting the chamber to its
Length and radius to suit cartridge used il a
-Forearm retainer screw; silver soldered in place; tap 8 x 40
inietted for trigger guard; screw hole to clear 10 x 32
inietted for trigger guard; screw hole to clear 10 x 32
-Forearm retainer screw; silver soldered in place; tap 8 x 40
Height as required 1.
proper depth, screw the barrel in place in the receiver with the extractor in its closed position. This will properly align the contact between barrel and extractor. Now the actual cutting is done by hand.
The lever is cut from 1/2 inch flat stock. It will require quite a bit of sawing and filing to cut it to final shape; just persevere and be patient.
Note that the lower end of the lever is cut to a width of 1/4 inch, with a 3/16 inch hole drilled for a pivot pin. Its upper end is also drilled, to receive a lower link pin. Cut a 3/16 inch wide slot for the link pin to fit in. A longitudinal recess 3/8 inch wide is cut in the horizontal part of the lever for the lower side of the trigger guard to fit into. This recess is made with either the edge of a flat file or with a small square file, except for the portion at the extreme rear end. Cut this area out with a chisel ground slightly narrower than the 3/8 inch slot width.
The latch, which is pinned into the outer end of the lever, is made from 3/8 inch flat stock. Trim its upper portion, which latches into the front of the trigger guard, to a width of 1/4 inch. Drill a hole for the pivot pin into the latch, and checker or groove the latch's upper surface. Also, drill a recess at the lower front of the latch for a suitable coil spring to fit into. This spring exerts upward pressure on the lever latch.
3/16 inch flat stock is used to make the link. Select steel that you know how to harden properly for the link. The leaf spring steel mentioned previously will work well for it. The link should be just thick enough to work freely in both the slot in the upper end of the lever and the slot in the breech block.
The 3/16 inch link pin holes must be as close to the specified distance of 1.250 inches as is possible, so that the link pin will cam "over center," and hold the breech block securely in its closed position. The resulting downward pressure on the breech block will actually force the lever tighter against the lower trigger guard, insuring that the firing pin will strike the primer in the same position each time the gun is fired.
Make the trigger guard from two pieces of 1/8 inch flat stock. A flat plate is cut to the shape shown, then slotted for the trigger, with screw holes drilled and counterbored front and rear. Drill these holes with a No. 18 drill to provide clearance for 8 * 30 screws. The forend retainer screw goes into the front hole, and a corresponding hole for the rear screw is drilled and tapped into the bottom of the receiver.
A guard bow is bent to shape as shown from a 3/8 inch wide strip of the same stock and welded to the plate. If the welds are built up slightly and the rounded fillets filed with a round file and polished smooth, a better appearance will be achieved. Note the slot in the right rear side. This is to provide clearance for the trigger bar. The rectangular opening at the lower front engages the lever latch and
6 x 48 Screw
To clear 8 x 40 screw
6 x 48 Screw
To clear 8 x 40 screw
ft clear trigger bar
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