All extant bullpups are trash. None of them are designed properly or with even modest levels of creativity. Almost all are some other design crammed up its own ass with resulting penalties in weight, size, balance, and other characteristics.
Well that's what it feels like sometimes to easily-frustrated people like me, anyway. It's just as well, as the bullpup concept only has very modest merits to begin with and they are slowly losing market share with military customers to more conventional designs.
Still! I think it's worth going down this rabbit hole all the way. Bullpups are best designed not as adaptations of conventional weapons, but holistically and from the start as such. So, let's do that.
So, in the time between the suppressor design that I am doing for work, I decided to go after a semi clandestine manufactured SMG. The criteria driving the design is that round tube is incredibly common in SMG designs, so I wanted to avoid that and use rectangular tube and bar stock. This minimizes the operations that require a lathe, and while they ideally would be done on a mill, they can be accomplished with a drill press and hand tools if you have the patience. Some of these parts are innocuous enough that they could also be farmed out to local machine shops without raising eyebrows. Strangely, one of the larger issues that has faced clandestine small arms manufacturing is an acceptable human interface. Other people solve this by traditional methods such as carved wood grips, or cast/molded plastics, but that is a time consuming process to make a part that should be very simple. My solution was to use handlebar grips from a bicycle. They are already designed to provide a griping surface for your hands, and they are common enough and varied enough that you could will not have a problem sourcing them.
As far as the design goes, it is still a work in progress. The receiver is pretty much dialed in, as are the trunnions, the barrel, barrel retention system, etc. The FCG has been a sticking points, as designing them is probably my greatest weak point when it comes to arms design. As the FCG is horribly incomplete, the bolt may similarly undergo changes. It is currently planned to have a linear hammer, but that is still in the works. I have only begun to consider what to do for the stock, and the forearm will probably come last. The design uses Uzi magazines, and I'm toying with the idea of being able to change magazine compatibility by having alternate lower receivers.
There are two primary versions; the 9" barrel original design, and the 5" barrel design.
Charging handle/bolt/action spring interface
Early receiver, designed to be cut out of 1x2" 11ga rec steel tube. The notches near the trunnion and front barrel support are to allow the components to be welded together.
charging handle and barrel retention system details. The action spring guide rod runs through the front barrel support and in conjunction with the receiver it locks the barrel retainer plate in place. The retention plate slips into a grove cut into the barrel. This prevents the barrel from moving backwards out of the receiver, while the square section at the breech of the barrel nests in the trunnion to prevent forward movement. The barrel is not rigidly fixed to the receiver, but this is acceptable considering the intended applications of the weapon.
Now that all the dust has settled on the F-4 project, I am beginning to think about taking up a project that I first considered a few years ago to re-engineer the receiver architecture of the AK-74. The concept here is to retain the fire control and moving parts group of the AK-74/100 series, but to replace the sheet steel receiver with a polymer unit that featured molded-in steel rails - which I call "glockerskates". A new upper receiver system similar to the AKB-15, to replace the upper handguard, rear sight block, and dust cover, is probably also called for, but right now I'm more focused on the lower receiver design.
The potential benefits of a receiver configuration like this is pretty obvious. A polymer lower receiver would be simpler and cheaper to make, and would require less labor to complete the finished product. No rivets would be required, and simple forged and stamped components - like front and rear trunnions and rails - could be simply molded in. Having said that, injection molding a receiver of this type would be a relatively complex operation to get right - multiple steel inserts would need to be properly aligned during molding, and the receiver design would have to be carefully engineered for optimum molding characteristics as well. Having said that, it doesn't seem impossible, so why not do a little design study?
I haven't yet done any work on this concept yet, but I do expect I'll begin work sometime soon, so look forward to that I guess.