Concept art Project 1657 of Frankensteined existing products.
The test fit AR57 upper to the M16A1 lower with the original Barbie outfit
The problems:
1) Buffer blues. Turns out I'm a slave to the buffer so anything bullpupped AND using a carbine tube has just too much length of pull. The AR57 upper is not a bufferless design. Worse (for this application), it is a straight blowback design. That means no lock or delay. The AR57 demands a heavy buffer mass to resist the initial rearwards travel of the spend cartridge. Other threads here and elsewhere show that bulged case necks are common from this upper and a certainty if the buffer mass is not adequate. Amping up the spring on a PCC setup does not overcome missing buffer mass and eventually reaches the point where the gun won't cycle due to over springing. So great, stuck with a buffer, how do I shorten it?
2) Semi-auto bolt. The last few generations of the AR57 upper come with a bolt that is needlessly cut for semi-auto only. The 'trip' is too far back to engage the sear or FRT if that is your game. Worse, the area in front of the 'trip' is milled shallow so it will rub/grind and damage the sear if used on an M16 lower. How to modify the bolt to work on my M16 lower. Fortunately, this road has been travelled.
[imghttps://imgur.com/sEpmj7x[/img]
M16 BCG top showing the trip ledge for the sear forward of where the ledge is on the stock AR57 bolt. Sharpie mark on AR57 bolt shows where the weld up needs to be. Note at the back of the semicircular slot for the hammer the AR57 bolt shows tiny dents where it interfered with the M16 sear during test fit.
3) No place for optics or irons on a bullpup. If you bullpup an AR57 upper, the main rail becomes your spot of cheek weld and the front rail section is tiny. I'll need to add a rail section above the magazine without compromising magazine access. P90 solves this with a bridge section above the magazine.
The requirements:
1) The lower receiver is sacrosanct. This lower receiver is the registered Class III item and cannot be modified in any way. The AR57 upper will have to attach normally and any bullpup pieces will need to be designed to sandwich the M16 lower as is.
2) The buffer and tube has got to be short. The buffer tube must be reduced to under five (5) inches in order to reduce the length of pull of the bullpup combo to under 15.5 inches.
Solutions:
Proposed buffer solution. Using an ARMASPEC SRS-9 heavy captured spring buffer for an AR PCC as a starting point, I will machine this down to minimal cyclic travel, ~3 inches. To do this, I will shorten the Delrin spacer at the rear and shorten the guide rod. I have a small CNC mill so this is an in house job. I believe the ARMASPEC buffer weight is not tungsten so I can modify it as needed. See bolt mods for the real craziness.
Stock ARMASPEC SRS-9 silent buffer system.
Proposed mods to the buffer system to shorten length, allow only needed travel, and travel partway into the AR57 upper receiver.
Proposed bolt solution. I will need to weld up the bolt and then machine it down to put the trip in the correct position to activate the sear. I will also need to ease the area between the hammer slot and new trip to prevent wear on the sear and bolt. This work has been done elsewhere effectively. I have a MIG welder and the aforementioned CNC so I can do this in house. There is no reason to reach out to Panzer to exchange bolts because of further mods. If you are not familiar with the Honey Badger system, they shorten the bolt and allow the buffer weight to travel inside the upper receiver, enabling a shortening of the buffer tube by moving part of the process into the receiver. I look to do the same by removing about an inch off the back of the AR57 bolt, putting a threaded hole through the back of the bolt and screwing the buffer weight directly to the bolt. My math shows I will retain the same additive mass of the bolt and weight as the original AR57 bolt and included buffer weight so it should work (opinion). Unfortunately, this solution does mean turning down the front portion of the buffer weight to the diameter of the AR 57 bolt which is considerably smaller than that of the AR-15 BCG.
Illustration of machining and mate up concept for integrating the buffer weight into the reduced-length AR57 bolt.
Proposed optics solution. I will machine something out of 7075 aluminum that matches the functionality of the upper on the P90. Mine will engage the tiny front rail and the side M-LOKs of the AR57 upper to give stability to mounted optics.
Anyway, that is the gist of the project. I plan to begin with modifications to the buffer weight first to see what I can or cannot remove from the back of the bolt. I will also do the basic mods to the AR57 bolt to get full auto function with the standard AR57 buffer in a carbine tube. From there I will make mods to the rear of the bolt to get the buffer weight to attach. The next tests will be with the mod bolt, mod buffer and mod tube to get min-length tube function. After that I will machine and print the bullpup parts.
I've included a lot of details to show what I am thinking and the problems I have thought about. What have I missed? I might have to add weight to the buffer if I ever try and suppress it.
Project 1657