Bullpup and Electric Trigger, Part 2

The wooden mockup chassis in Part 1 was a proof of concept build for the electricallyactuated trigger system. The aluminum chassis is a more practical design with better ergonomics and parts fit up. It resembles a real bullpup rifle, as opposed to looking like a wooden rifle case that left the barrel sticking out and had a funny looking handle. With that prep work complete, I moved on to building the aluminum chassis.

I decided to use a readily-available aluminum stock for the build to minimize the amount of machining required. The 1.50" wide channel with 1.00" high walls and 0.125" wall thickness was a natural choice for the major component of the chassis. The nominal 1.25" inside width of the channel was very close to the 1.256" receiver width Top: Aluminum channel with the barrel support on the right end.

The hole for the action screw is on the right side, the four mounting holes for the solenoid are on the left. and required milling only a few thousandths on each side to provide a close tolerance fit for the receiver. The channel ended up being 25" long.

I made a template of the opening in the bottom of the Ruger stock for the action and trigger group (referenced from the action screw location) and transferred it to the aluminum channel and There are a few extra holes due to my not paying attention when marking the hole locations. The barrel support screws are on the right. then milled out the contour. I determined the correct location for the solenoid and drilled the holes for the mounting screws.

I used a 10/22 barrel that I had removed from a previous build, shortening it to 16.5" and then chamfering and cold bluing the muzzle. It was installed into the receiver and the barreled action set into the chassis. The barrel was not quite aligned with the centerline of the chassis, so some additional fitting was required to center the action horizontally. I leveled the action to the chassis and measured the height for the barrel support at the front of the chassis.

I machined it from the 0.75" thick aluminum plate that I also used for the pistol grip. It’s attached to the chassis with two 6-32 thread screws. As I mentioned in Part 1, the 10/22 receiver only has one action screw and the barrel is supported at the front of the stock with the sides of the receiver supported in the stock

as well. After installing the barrel and receiver supports on the chassis, I bedded the action. The barrel support filled in most of the opening on the front of the chassis to help keep out debris. Pistol Grip The pistol grip on the mockup was too small due to the shape of the 25round magazine and wanting the length of pull (LOP) to be between 14" and 15". I measured the LOP on Below center: Cordless tool battery as purchased. a few of my rifles and found that it varied from 13" to 15.5".

The 15.5" LOP was comfortable so I decided to increase the bullpup LOP to 15.5" to allow a larger pistol grip and still be able to use the 25 rounder. A 15-round mag is available and its shorter length provides additional clearance. I machined the outside of the grip to the desired shape and then milled the interior to reduce the weight and accommodate the trigger micro-switch.

I thought about including the grip safety but decided it was not needed as the 10/22 cross bolt safety was easily operable with the aluminum chassis. The pistol grip required a few setups on the mini-mill and I got a lot of practice making chips. I radiused the front and rear of the pistol grip and installed 1911 style grip panels to add a little width and make the grip more comfortable. I made the trigger from 0.25" thick steel. Its pivot pin is inside the chassis as on the mockup.

I considered making the pistol grip longer vertically to contain the trigger pin but the added length would have interfered with the magazine. The trigger and pin are contained in a separate block which is mounted on the chassis. Exterior Shells I used 0.040" aluminum sheet for the exterior shells on the top and bottom of the chassis. The shells had left and right side L-shaped sections with a 30 degree transition from the vertical to the horizontal plane.

When assembled, the horizontal sections overlapped, doubling the thickness and increasing the stiffness. My sheet metal brake

was not wide enough for the longer pieces so I had them made at a local shop. I chose to bond the left and right sides together at the horizontal sections as opposed to welding to prevent heat distortion. I drilled and tapped the channel with 6-32 threaded holes and attached the shells to the channel with button head screws. I added a Picatinny rail section to the upper shell to mount optics.

A vertical cutout on the right side of the upper shell was required to allow it to slip over the bolt charging handle during assembly. The front and rear openings on the lower shell were covered with the 0.040" aluminum sheet. The rear cover of the lower shell acts as a buttplate while the rear cover on the upper shell allows access to the battery. All the external edges were deburred and the corners rounded to protect the shooter’s hands.

The upper shell fits closely to the re- Fortunately, the battery fit snugly into the upper shell and did not require any retaining clips. I had intended to plug the battery into the rear of the stock like on a cordless tool. I wanted the battery to be fully enclosed in the stock as opposed to a cordless tool where the battery was readily accessible.

Trying to orient the battery so that it would line up with an internal plug proved to be impossible, so I decided to insert the battery into the stock and then plug the connecceiver and has a much slimmer profile than the wooden mockup. This allowed a better cheek weld and eliminated the need for the riser when installing optics. A thin sheet of textured rubber was added to the buttplate. A foregrip with radiused edges was added to the bottom of the chassis forward of the pistol grip.

I painted the upper and lower shells with flat black paint but left a section of the upper shell and the pistol grip frame unpainted to give some contrast. Apiece of neoprene was added to the left side of the upper shell as a cheek pad. Battery Wanting to reduce the weight of the bullpup, I kept looking for a lighter battery.

I found a Bosch cordless tool battery that weighed only 5.6 ounces compared to the 12.9 oz. lead acid battery that I used in Top: Assembled bullpup with 15-round magazine and red dot installed, right side. the mockup; a reduction of almost half of a pound. The Bosch battery had a triangular cross-section, about 1.5" across the lobes after I removed the grip cap with a few careful cuts with a hacksaw. It was slightly shorter at 3.25" compared to 3.75" for the original.

1 Barrel 2 Barrel Band – Barrel Band Nut 3 Barrel Band Screw 4 Barrel Retainer 5 Barrel Retainer Screw – Barrel Shroud 6 Bolt 6A Bolt Assembly 7 Cocking Handle 8 Bolt Lock 9 Bolt Lock Spring 10 Bolt Stop Pin 11 Buttplate – Recoil Pad 12 Buttplate Screw – Recoil Pad Screw 13 Disconnector 14 Disconnector Pivot Pin 15 Ejector 16 Escutcheon 17 Extractor tors into it.

To remove the battery, the end cover of the upper shell is removed, the connectors are removed and the battery drops out when the bullpup is held vertically. The solenoid requires very little current when actuating the trigger as compared to a cordless tool so the battery will stay charged for months. Removing the battery disables the bullpup unless the lower shell is removed. I used soldered connections to reduce the number of connectors.

The wires from the 18 Extractor Plunger 19 Extractor Spring 20 Firing Pin 21 Rebound Spring 22 Firing Pin Stop Pin 23 Front Sight 24 Hammer 25 Hammer Bushing 26 Hammer Pivot Pin 27 Hammer Spring 28 Hammer Strut Assy. 29 Hammer Strut Washer 30 Magazine – Magazine Cap – Magazine Cap Nut 31 Magazine Latch 32 Latch Pivot/Ejector Pin 33 Magazine Latch Plunger 34 Latch Plunger Spring – Magazine Release – Magazine Rotor – Magazine Rotor Spring Ruger 10/22 (Speci ications subject to change without notice) – Magazine Screw – Magazine Shell – Magazine Throat 35 Rear Sight 36 Receiver Cross Pin – Receiver Filler Screw 37 Safety 38 Safety Detent Plunger 39 Safety Plunger Spring 40 Sear 41 Sear Spring 42 Stock – Stock Medallion 43 Takedown Screw 44 Trigger 45 Trigger Guard 45A Trigger Guard Assembly 46 Trigger Pivot Pin 47 Trigger Spring Plunger 48 Trigger Plunger Spring – Auto Bolt Release – Flash hider – Front Band – Muzzle Cap – Muzzle Cap Escutcheon – Muzzle Cap Screw – Muzzle brake – Pistol Grip Cap – Grip Cap Medallion – Pistol Grip Cap Screw – Spring Stop Pin – Scope Base – Scope Base Adaptor – Scope Base Screw – Sing Swivel, Front – Sing Swivel, Rear – Sing Swivel Base – Sling Swivel Plate – Sling Swivel Screw – **Unnumbered Items Not Illustrated trigger switch to the battery and solenoid are routed along either side of the receiver.

At the Range The aluminum chassis version was more comfortable to handle and shoulder than the mockup and it was better looking; none of these facts are surprising. The aluminum chassis with the pistol grip, battery, and solenoid weighed 2.80 pounds. The outer shells with the optic rail mounted weighed 0.98 lb. The barreled action weighed 2.81 lb. without a mag. Overall weight with an empty 10-round mag was 6.72 lb. and the overall length was 26.25".

This compares to 6.44 lb. and 29.5" for the mockup which had a 20" barrel. I shot the bullpup at 50 yards from the bench and standing. I installed ared dot optic to keep the overall weight to a minimum and ared dot seemed more appropriate for this type of rifle. On the bench, it averaged 2.16" for three, five-shot groups. The standing groups averaged 6.23". Not quite as good as the mockup that had a scope.

After completing the accuracy tests, I started plinking at the steel plate at 50 yards and just enjoyed the fun of shooting a .22 rimfire rifle. The bullpup functioned without any problems in typical 10/22 fashion. The electric fire control system provided a very light and crisp trigger pull that is typically not found on bullpup rifles, or most other firearms for that matter.

After putting about a hundred rounds through the bullpup, I switched to my Ruger GP100 wheel gun which has a decent single action trigger pull of 4.5 pounds. The GP100 trigger felt very heavy after shooting the bullpup. It took several rounds to adjust to the GP100 trigger again. The major drawback of the electric trigger system was obviously the weight and size of the solenoid and battery.

Designing a trigger system specifically for a bullpup using this concept would likely result in a lighter and smaller solenoid and battery as compared to the "off the shelf" components that I used with the standard 10/22 fire control group. This was an interesting project; it challenged and improved my design and fabrication skills. Some may say this project was an answer to a question that nobody asked. Maybe so, but it kept me entertained during those cold winter months. AG