Introduction: Powerline 880 Air Rifle Bullpup Conversion

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... 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.

Step 2: Pistol Grip

I figured it'd be easiest to epoxy the pistol grip in place once everything else was put together, so that's what I describe in this step.

The grip its self is simply a pleasingly-shaped block of poplar with a 1/4" hole drilled through it as close to the top as you are comfortable with - I left about 1/4" of material above the hole.

I cut the top off a piece of 3/4" box section to create a piece of U-channel about 4.6" long. The tail of this is bent down to match the angle of the hand grip on the pump arm. (people sourcing their U-channel from other places will find it useful to know that the cross-section of the region of the pump arm we are attaching to is 0.625" wide  by 0.42" tall)

Calculations showed that the pump arm only had a factor of safety of about two, so I didn't want to mount the U-channel using big bolts; towards the front, where the safety factor was lower, I drilled a tiny hole and just put a little pin through it. Both of these holes were drilled through the groove in the pump arm, where they'd be close to the neutral axis and remove less material. See picture for details.

Drill and file to create holes through the hand grip on the pump arm which are in line with the hole in the block of poplar, and big enough a straight rod (Ø about 9/64") passes through all three with a bit of play - maybe 1/16" in each direction.

Using this rod to make sure you don't screw the alignment up so bad the whole thing doesn't work, attach the metal to the pump arm and epoxy the pistol grip to the metal.

You might be tempted to try to simply epoxy the pistol grip to the pump arm - don't. The pump arm flexes too much and is made of nylon, and the PG will come off.

Step 3: Trigger

The trigger rod is bent at one end as shown. The first, sharper, bend is so that it follows the angle of the pump arm hand grip; the second, shallow, bend near the tip is so that the trigger rod does not have a tendency to slide up or down on the trigger shoe, which could make the trigger pull nasty.

Polish everything nicely so that it runs smoothly.

The trigger is mounted by heating everything up with a propane torch, applying hot glue to the trigger rod, and assembling. This allows the position of the trigger to easily be adjusted and for the trigger/trigger rod to be disassembled.

Step 4: Buttstock

The buttstock is made from from a piece of poplar 1.26" thick and two pieces of (0.1" thick) 1.5" SDR 21 PVC pipe, flattened by heating in the oven and smooshing between two boards..

This is simply epoxied together on the left side (the side which contains the molding for the trigger guard).
On the right side, the connecting piece of PVC is epoxied to the reciever plate but screwed to the poplar; this lets us disassemble the receiver if we ever need to in the future.

A trigger overtravel screw mounted in a counterbored hole, such that you can adjust it though a hole in the buttplate, greatly improves the trigger pull.

Step 5: Scope Riser

If you got a welder, this is where to pull it out. I don't (yet!), so everything here is done with epoxy.

The scope rail is two pieces of 1/2"x1/8" steel bar, one with 30 degree undercuts, epoxied together.

The riser proper is made from a shelf bracket with the top (which was fairly round) squared up a bit so that it'd present a better surface to epoxy to.

I assembled the whole thing as follows:
  1. Sighting in a laser sight mounted on the original scope rails.
  2. Gluing the scope riser on
  3. Mounting a scope to the scope rail and centering the dials on the scope
  4. Gluing the scope rail to the riser, lining up the scope's point-of-aim with where the laser was pointing

Step 6: Finishing and Use

At this point, I chose to apply a cote of polyurethane to the wood so it didn't get really dirty, but otherwise...

Remove the scope, cut a hole in the forearm so that it allows the barrel and scope riser to float freely inside, assemble everything, and you are done.

To use, just move the trigger rod forwards until it sticks to the magnet, pump, load, aim, pull the trigger back until it parks its self in the trigger shoe mounted to the real trigger, and then pull the trigger as you normally would.