Singl Barrel 12ga Gun Pin Hammar Parst

Cut from 12 ga sheet. Fold on dotted lines.

Cut from 12 ga sheet. Fold on dotted lines.

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Trigger housing pattern.

Cut from 12 ga. Sheet, Folded To Shape


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Closed Bolt Hammer Pin Location

Closed Bolt Sear Pin Location

Fal Type Receiver Sear Cut

Drill Locating Pin holes with Housing in Place in Receiver to Assure Match.

Trigger housing.

Spacer, .180" Thickness


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Dimensions are Approximate. Must be fitted during Assembly.


Sear Cut Ar15


Closed-bolt sear.

.250 Spring

Pocket .250 Deep

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.250 Spring

Pocket .250 Deep



Spring Guide Pin is .125 Drill Rod pressed into #31 hole.


I Open-bolt trigger components.

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SEAR CATCH Dimensions are Approximate.

Fitted During Assembly.



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Hammer Spacer is pressed into .217" hole through Hammer Use ML6/AR15 hammer Spring.

Closed'bolt trigger components.

Coil Spring

.187" Spring Pocket .550" Deep. Detent .050" is turned to fit inside Spring from .187 Drill Rod.


Dimensions are approximate.

Must be fitted during assembly.

An end cap, or cover plate, is turned as shown and welded in place at the top rear. This area receives a considerable amount of jolting and pounding bccausc the rccoil spring bears against it and force is exerted against it each time the bolt travels to the rear. This is the reason for the built-up fillets along the lower sides. The extra length when welded in place reinforces and stiffens the joint. The upper half is cut away as shown, leaving only the thicker section at the rear. This enables the receiver to hinge upward when the grip bolt is removed, allowing bolt removal and access to the trigger assembly.

Both the upper and lower receivers should be clamped together and checked for proper fit. Assuming they fit together without gaps or cracks, drill and tap the front hinge bolt hole. This is done through both the upper and lower receivers at the same time, with both clamped together. Make the counterbore for the screw head at the same time. With the hinge screw in place holding the forward end together, drill and tap the grip screw hole. With the grip bolted in place, this forms a solid, sturdy assembly.

Although it would be easier to drill through the sides of the lower receiver for the various pivot pins required in the trigger mechanism, I designed this gun to use a separate trigger housing that contains all the trigger parts as a unit and is contained inside the lower receiver with no exposed pins. This will take a little bit of extra time, but it is time well spent since the pins are contained and cannot work out in use. It also has the advantage of interchangeability, allowing both the open- and closed-bolt assemblies to be used in the same frame.

Cut a blank for the trigger housing to size and bend it around a form block, just like we did before. This one is made from 12-gauge sheet and must fit closely inside the frame. Cut a slot for the trigger in the bottom, as well as a slot for the safety when the closed-bolt version is used. Matching clearance cuts must be made in the lower receiver to accommodate these. You should also drill holes for the pivot pins, using the locations shown in the drawings. If 1/8-inch pins are used, drill through both sides with a #31 drill. This is followed, through one side only, with a 1/8-inch drill. The smaller hole made by the #31 drill will grip the pivot pin and hold it in place. Hinge pin holes through the component parts should be drilled with a 3.20mm drill that is .126 inch in diameter to provide clearance, since a 1/8-inch pin will not revolve freely in a 1/8-inch hole.

The open-bolt trigger assembly uses a two-stage trigger pull instead of a selector switch. A short pull of the trigger fires single rounds only, because the disconnector disengages the trigger when the bolt, as it moves forward, cams it downward. Pulling the trigger further to the rear, against the stiff spring, causes the second notch on the trigger ba i lo bear against the sear lever, holding it out of engagement. This allows the bolt to reciprocate unimpeded, causing full automatic fire as long as the trigger is held back.

The sears, hammer, and sear lever should be made from 4140 or equivalent. Leaf-spring material can be used. These must be hard enough to prevent wear, but not so hard as to be brittle; 4140 hardened and the temper drawn at 800 degrees will be satisfactory for this.

The trigger bar should also be made from material that will heat-treat. You may have to mill or saw leaf-spring material to the thickness required to obtain it. Don't try to use common sheet metal for this. Triggers can be made from any available steel.

The trigger guard is simply a 1/2-inch or 5/8-inch-wide strip of 14-gauge sheet metal, bent to whatever shape suits you and welded or silver-sol-dered in place.

The sliding safety, as used in the closed-bolt gun, is made from 3/8-inch-thick material to the dimensions shown. When properly fitted, a flange at the forward end slides under the tail of the sear, locking it into the face of the hammer. The only way it would ever fire with the safety engaged would be to break it.

The open-bolt handle latches in the slot cut in the handle raceway. Here again, you would have to break it to allow it to fire when engaged. It would be a good idea to go ahead and cut the safety slot even when building the closed-bolt version, in which case it will serve as a bolt hold-open device.

If you choose to make both bolts and both trigger assemblies, they can be interchanged in the same frame and receiver, provided you cut sufficient clearance in the bottom of the receiver to clear both hammer and sear, as well as both disconnectors.

Left: Sheet-metal sides are cut using a band saw.

Below: Flanges can be formed in vise by using a heavy hammer and form blocks.

Below Left: Lower receiver bhnks cut from sheet metal.

Sheet Metal Receiver

Left: Sheet-metal sides are cut using a band saw.

Below: Flanges can be formed in vise by using a heavy hammer and form blocks.

Below Left: Lower receiver bhnks cut from sheet metal.

Machinist Drawing Lower Receiver

Left: Front face of lower receiver ready for welding.

Middle Left: Seam along lower side is also welded. Note that the filler block is in place while welding.

Middle Right : Rear end cap is welded in place in lower receiver.

Bottom: Welding is performed with TIG welder.

Ralph HardySheet Metal Pistol ReceiverWho Welds Rear End Housing End Caps

Above: Front hinge block is welded at the front of lower receiver. The weld should be built up to allow fillet to be formed.

Right: Receiver is contoured to mate with upper by using ball cutter.

Below: End cap is fitted and welded in place.

Fillet Feld Grind Smooth

Grind welded joint, file smooth, and cut upper portion Ready for final finishing, away to allow receiver to hinge upward.

Left: Closed-bolt trigger assembly consists of trigger, trigger Ixir and spring, sear, hammer, and safety.

Below: Component parts for closed-bolt trigger assembly fit inside removable housing. This is interchangeable with open-bolt assembly.

Open Bolt Machine PistolOpen Bolt Machine Pistol

Left: Open-bolt trigger assembly, shown as assembled.

Below Left: Hammer uses M16/AR15 hammer spring.

Below Right: Parts, when assembled, make up an interchangeable assembly.

Pistol Sear Spring AssemblyTrigger Spring Correct Ar159mm Far PistolOpenbolt Sear And Trigger

Top Left: Sear as used with the above hammer.

Bottom Left: Safety as used with the above.

Right: Closed-bolt trigger with trigger bar and spring in place.

Open Bolt Machine PistolSpring Bolt Machine Pistol

Far Left: Top view of closed-bolt assembly.

Left: Top view of open-bolt assembly.

Far Right: Bottom view of same.

Right: Closed-bolt trigger assembly (left) and open-bolt assembly as viewed from top.

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