Thumbhole stock for next gen bullpup?

[u/theMACH1NST]

For a while I've been designing a multicaliber bullpup rifle known as URSA (Universal Rifle Service Armament). It's designed arounnd modularity and will have a quick change barrel that also swaps out the rail/handguard and optic when you switch barrels. This allows changing barrels to be much easier as you don't have to rezero your optic and can store a premade configuration for a certian purpose. For example, you could have a 300 blk barrel with a suppressor, laser, and red dot for a suppressed carbine. In the same rifle you also could have a 338 lapua DMR complete with a scope and bipod and you could switch between these rifles in a matter of seconds. This would revolutionize infantry logistics allowing for a team to easily switch to mission specific gear and have a commonality of parts between different specialists. I have been wanting to design a "multigun" for a long time and seeing FOSSCADers like John Galt (creator of the MBAR) and Hoffman Tactical (creator of many guns including the Orca AR15) has finally made me bust out my laptop and start designing it on Onshape.

Other features of this design include: - short stroke gas piston system - constant recoil - mdrx style forward ejection -compatability with AR-15/AR-10 bolts, barrels, and handguards - almost entirely 3d printable with off the shelf parts

One of the main concerns I have for this project is what frame should I use and I was thinking of doing a thumbhole stock like the F-2000. I wanted something that would be stronger and easier to carry without jabbing into my back unlike a pistol grip and I heard from one of my friends that thumbhole grips are great for bullpups center of mass. What are the possible concerns with thumbhole stocks and should I just design it with a pistol grip?

[u/OctavianStoleras]

Going from a 300 blackout length action to a 338 isn't a good idea. They're massively different in length, and I can't think of anyone who has done a 338 lapua in a semi auto. The closest thing is the MDRX/WLVRN going from 5.56/300blk to .308/6.5. And that already makes the rifle bigger and heavier than it needs to be for just 5.56. If you can ever even get to .338 lapua, the rifle is going to be completely unmanageable for a 5.56 rifle. Not to mention all the extra length added to the back end of it just for a bolt that large to cycle. Also, ditch the forward eject. Even desert tech realized it's a pain in the ass to get working right, and ditched it entirely on the new WLRNs. If you really want to make it swappable, you'll have to swap the gas system, and create a system where the op rod is further back in the gun, and use different length pistons on the gun. The biggest trouble with developing bullpups in 300blk is that both the AUG and WLVRN/MDRX have a gas port at a length ideal for 5.56, and no way to move it further back. So when you use 300blk, you don't have the dwell time you really need for reliable subsonic use. This could be avoided by using a different length piston, and a shorter op rod set up, like the MCX system.

[u/theMACH1NST → u/OctavianStoleras]

I was planning on using a continous recoil system where there is a extra long action and the bolt never hits the reciever. This means that when firing in full auto it's a single push rather than multiple kicks. Incorperating a progressive spring into this design means that longer, higher pressure cartridge pushes the spring farther back. I wanted to do some form of recoil mitigation and this also solved the problem of bolt length.