Wallydog21 wrote:Morning all
Im really interested in the bullpup chassis, how and where did you start? Whats the trigger assembly like?
cheers
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Two identical Ruger American Predators, top in .223, bottom in .204.
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To swap a barreled action in simply requires removing the butt plate (one screw), the lower cover (five screws), and the recoil lug (the original action screw). Slide the action into the tube, drop the recoil lug into the slot in the tube and screw it to the action - this locks the receiver axially and rotationally into the tube, and transmits recoil into the tube wall. Drop the lower cover on the bottom of the tube, ensuring the original trigger falls into place behind the transfer bar. Insert a screw into each side of the recoil lug, and one up into the tube at the front. Place the front cover into the opening and fix with two screws. Install the bolt. The butt plate clips into a slot in the top of the tube, folds down and is held by a screw in the side - this allows very easy bolt removal. This does not allow me to keep the scope on the barreled action, as I have to remove the pic rail from the rifle. Leaving a scope on the bullpup chassis maintains a very close zero when the action is dropped in though.
The only electrical tool I used was a cordless drill. Everything was cut using a hacksaw and a coping saw and finished with files. The countersunk stainless screws are all the same pitch (cut to the required lengths), so only one tap was required.
For my Ruger Americans, it started when I discovered there is a standard aluminium extrusion that happens to be exactly the same ID as the OD of the circular Ruger receiver - from memory I think the tube is 40x3mm. This means no machining required. Simply slide the barreled receiver into the tube and fix it in place, then hang the peripherals off the tube. The fit is very close, but not interference. I simply cut a slot in a rod and put some emery cloth in it and used a drill to clean the inside surface of the tube of any burrs or defects. It would have been nice to have a little clearance though as a couple of strips of insulation tape would prevent the receiver rubbing against the tube, not that it matters.
Then I cut a slot in the bottom of the tube to fit around the trigger pack. I cut this in far enough that the bolt can function without hitting the butt plate. I could shorten the rifle further but would not be able to operate the bolt while it is shouldered as the bolt would have to pass through the butt plate. As my barrels are 22" and the rifle is 860mm from muzzle to rubber butt pad, an eighteen-inch barrel would shorten the rifle to 760mm while still allowing bolt operation. Putting a Ranch Rifle into this chassis would be illegal due to the 16" barrel.
I made a new recoil lug out of 10mm aluminium plate that is much deeper than the original. This piece was the single hardest part of the build and took hours to file it to the perfect shape and angle to perfectly tie the receiver to the tube with not even a hairs gap to allow any movement. Next time I think I would just order the steel V-blocks from MDT and weld an extension to one.
Next I cut out the ejection port, and two small ports to allow inspection of the original serial number, and operation of the bolt release. Then a long groove for the bolt to run in. It was my intention to have just a small cover over the trigger pack and hang the grip directly under the tube, but I was planning a ten-round magazine and felt the lower cover would better support it. As it also gives me somewhere to hide the trigger transfer bar I decided to leave it long, at least for initial testing as I had planned an electronic trigger. I was waiting on the solenoids though and really wanted to test it, so I bashed together a transfer bar in an hour one night, it works surprisingly well so I haven't altered it.
The lower cover is an RHS fixed against the bottom of the tube. I cut a slot in the top for the recoil luck to drop through, and a slot for the magazine well in the top, but not the bottom as I hadn't designed the magazine yet. A short screw fixes it to both sides of the recoil lug, with a third screw at the front up into the tube. The V gap left on each side I covered with a simple length of angle screwed onto the top of the lower cover. Then to fill in the front end I made a piece that clips into the bottom, folds up against the tube, and is fixed with a screw each side. My intention was to drill these through and use an AR15-style captive pin in place of the screws.
I made an AR15 grip tenon and screwed it to the lower cover to allow use of any AR15 grip, and made a trigger guard out of a piece of RHS.
The butt plate is cut out of 5mm plate to fit a standard M4 rubber butt pad. It fits neatly into the rear of the receiver tube and the lower cover to tie everything tightly together. It clips into a slot in the top, then folds down and into the tubes, and is fixed by a screw on the left side of the lower cover. The intent was to replace the screw with a sprung detent pin.
The aluminium trigger hangs on a roll-pin pivot and presses against the transfer bar, which is simply a length of 25mm SHS that slides inside the lower cover, and presses against the original Ruger trigger. The transfer bar is cut around the recoil lug and magazine well.
For sighting I fixed some cheap Chinese Picatinny rails the full length of the top (to allow use of pic rail iron sights), and bottom and sides of the fore end to allow fitting standard rubber pic rail covers for a comfortable grip, and allow me to try a variety of components. The best iron sights are the AR15 carry handle and AR15 pic-rail front sight, very comfortable to use, and the scope mounts on top of the carry handle. Back-up sights offset to the right side though work fine also.
For feeding I made a simple ramp that fits inside the lower cover. It allows me to toss a round in the ejection point and feeds very cleanly and quickly.
I had already been working on a magazine to replace the original Ruger rotary mags (I don't like rotary mags, even when they work well). I fabricated a block that clips into the original poly stock in place of the rotary magazine. And I made a ten-round single-stack mag that simply clips in and out of the dummy magazine. But you can remove the dummy block as if it were an original magazine, and fit in an original mag with no modification at all - straight swap. Very handy to be able to use the rifle while trying get a new magazine design to feed properly.
I had trouble getting the new magazine to feed reliably and decided it might just be better to adapt it to take a commercial magazine. I was looking at the AICS mags when I discovered the MDT LSS chassis. Although the chassis was about $500 it gave me a ready-built magazine well. I bought one to cut up for my version-two of the bullpup. But before cutting it up I thought it would be silly not to at least try it out first. So I dropped the 7mm-08 into it, and loved the result. I have since bought six more MDT LSS chassis, including two for my rimfire Rugers as well. So, I never completed the version-one magazine system and just use the bullpup as a single-feed rifle.
I cobbled this thing together over a few weeks while sitting at the computer. I wanted to test the basic tubular design before building an actual chassis, but pretty much had to complete it to be able to test it. I took the original rifle out, shot some groups, removed the scope and rail, swapped the barreled action into the chassis, and shot some more groups. No measurable difference at all - amazing. I have put several hundred rounds through it and never had any issues at all. I usually keep a round or two in my left hand, which I can grab with my right hand and throw them into the action very quickly with the rifle shouldered.