Bullpups with the trigger group behind the trigger, using a trigger rod, are famous for having bad trigger pulls. This mod actually improved the trigger pull (lighter, smoother, adjustable for overtravel), while coping with the fact that the gun has an underlever right where you want to put the pistol grip.

Accuracy is approximately as good s it was pre-mod (about 2 MOA @ 20 yards), though it's been in storage in a humid basement for a few years after the old stock broke and seems to have developed a pickyness WRT ammo

Overall, a build using inexpensive materials with a fairly low tool requirement.
All replacement components are fabricated from poplar, steel, or PVC and held together with epoxy or screws... plus a single rare earth magnet, and a tiny bit of aluminum and hot glue for the trigger. (Total cost for materials used was about 10$.)
You'll need a vice, a hacksaw, a drill, a set of files for wood, a set of files for metal (or a Dremel and a bench grinder), and some way to make boards of a specific thickness. A laser sight would be a real convenience, and a band-saw would make some steps a lot faster.

Because of the number of details in the project, I won't be doing a step-by-step for the entire project, just for a couple tricky bits. I will try to provide good pictures and descriptions to let you know what design-type stuff I came up with.

Anyways, onwards...
...to prime you, due to geometric dependencies, I recommend building the parts in the following order:
  1. Remove old stock. (Screws? Unscrews! Iron sights? Hacksaw!)
  2. Make forearm, except for scope-riser hole. (such that it fits in the receiver)
  3. Make Pistol-grip (such that it doesn't bang into forearm)
  4. Make Trigger group (such that it works inside the pistol-grip)
  5. Make Butt-stock (such that it gives you the right length-of-pull to the trigger)
  6. Make Scope Riser (such that it puts the scope in the right position)
  7. Finish forearm (with a hole that allows it to fit over the scope riser without touching it)

Step 1: Forestock

I stamped the U-channel for the forearm out of 1.5" sch 40 PVC pipe.

First, you'll need to set up your punch and die:

The punch is simply a piece of 2x4 that we machine to a thickness of 1.125". I did this by making a whole bunch of cross-cuts with an old-style radial arm saw with adjustable depth-of-cut; you use whatever you have on hand that'll work. This thickness determines how well the forearm fits into the black plastic of the receiver and onto the forwards part of the pump housing/receiver, so you want to do a fairly good job, say plus/minus a couple hundredths of an inch.

Conveniently, because of the thickness of the pipe we're using here, once we have a 1.125" thick board (with bottom corners rounded a bit so they aren't sharp), we can just set up a die cavity by clamping some 2x4s together in a vice.

Slit the PVC pipe lengthwise, warm it up in the oven for a little bit (3-5 minutes at maybe 300 degrees F), slip it onto the bottom of our punch, pop it back in the oven for a bit longer (5-10 minutes at 325*F seemed to work), and quickly take it over to your die and force it into the cavity with the punch. Now tighten vice a bit (squeezing the side boards in a bit), sit a weight on top of the punch, and go do something else while everything cools for 10-30 minutes.

When you get back, take the U-channel out of the die, true it up dimensionally, and shape one end so that it mates nicely with the factory-made plastic receiver (I've included a sketch to help you).

You'll need to locate a hole into which the forearm retaining screws go. This is a bit difficult, but with my tools (only calipers), I found it best to measure lengthwise from the receiver to the hole, then plug the forestock into the reciever, and draw an arc with this measurement as the radius. The intersection of a line about 1" above the bottom of the forestock and this arc is where I drilled the hole.

The bottom of the forearm is a piece of poplar held in place with short wood screws.

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