Got some work done, but some parts will need to be redone. I had a feeling going into it that there would be a lot of trail and error, and unforeseen issues might come up (and they did).
After a lot of eyeballing and calculating, I cut the tower off the FAL trigger housing, and milled out most of the back where the take down plunger and stock tang goes, and then welded it with a couple tacks incase I needed to make adjustments, onto the RPD lower (which at this point barely has anything left from previous undocumented attempts to fit alternate trigger groups).
After It was in place, I drilled out the hole for the bolt plunger to 5/8", but higher than the center of the original hole (top of the hole is just below the take down pin channel). Then I went to install the trigger group of the FAL with the hammer I had built up with weld.
The safety lever cannot be installed because the lip of the RPD lower is in the way. I'll have to work that out later.
Another issue that came up is the charging handle could not move back all the way because of the tack welds, which would be trimmed down and cleaned up when it was finished.
I switched gears and started working on the extension for the recoil spring tube, which the hammer would have to swing through the middle of to hit the firing pin.
It started as a 1" rod of 4140 prehard. I had spun most of it down to 5/8", recessed the tail to fit inside the tubing for the recoil spring, and cut a recessed ring in the back for the spring to seat against.
I started by machining the head that would mate with the bolt carrier down flat, almost flush with the 5/8" part, which would be the bottom.
This is how it fit in the lower.
Then I trimmed the sides to fit inside the receiver walls.
After comparing the parts for a while, I decided to mill off the bottom part of the bolt carrier at the back, just short of where the pin for the top cover roller was installed, and then trimmed the front and top of the spring tube extension to fit. This way, when I cut the slot for the hammer to swing through the extension, it is still held together at the nose, and the hammer can hit the back of the firing pin extension flush.
The camera drains batteries like a vampire, and my cell phones camera sucks, so I didn't get any photo's of cutting the slot for the hammer, but the rest of these shots illustrate where it stands and what the next steps are.
This is the parts laid out. The spring tube has a lip that will stop it from moving forward when the lower is removed, but it will be able to move alittle further forward than when the bolt and carrier are fully in battery. This way, that lip will not be under any contact unless the lower is removed, and has constant pressure on the bolt carrier when installed. The spring stops at the back of the slotted extension at about the same spot as it is now.
This is how the hammer swings through the slotted extension. I machined the slot 3/8" wide, and had to trim about .04" off each side of the hammer to fit.
I will also have to slot the bottom of the extension head, as right now, the hammer catches on it and cannot drop into the trigger housing any more without hitting the hammer spring tube or overextending and actually locking itself down under the pressure from the hammer spring. If I could angle the back of the FAL trigger housing lower off the RPD frame, it would solve that problem, and allow more room for the safety to fit (which will either get shortened/modified, or installed in a different clock location, like 9 instead of 12).
So, after locating a thick walled piece of 1" x 1.5" rectangular tubing, it appears that I can make a new lower to weld on. The dimensions are close enough, and even though the walls are thicker, I can always trim them down if I need to. This will also solve the issue of filling the curved are at the front of the FAL trigger housing.
I still have to work out the stock,... the wife wants a G3 style stock over an AR carbine extension. Either one will work, there is enough space inside both for the spring to fit now that it extends into the receiver.
I'll update when I make more progress.