Here is current recoil system discussion:
Here is gas system discussion:
You solve that, and you'll only have 99 problems - but your Big AR function won't be one of them.
Recoil System first, because it's SO FUCKED, on this platform. Gas system problems second. In that, gas tube length is primary these days, and gas port diameter is secondary. Gas systems on this Big AR platform are fucked, too.
Here's more recoil system information, that's recent:
When comparing recoil systems, for Carbine Receiver Extensions ("collapsible stocks"), there are two buffers that can be used, but the systems cannot be interchanged. You go ONE WAY, or you go THE OTHER WAY. Here's the data on the short carbine buffers:
You solve those problems - that manufacturers cannot possibly grasp - and your gun will run.