Well without the OP giving us any more info, we could guess until we turn blue..........
still seems like all you can do. unknown ammo, unknown barrel, etc. unless I missed it we don't even know if it's a std pinned A-frame FS or a low-pro gas block.
asside from checking alignment as you mentioned, unknown barrel could just have a small gas port. going to a carbine buffer vs H1 may help as well as going to hotter ammo.
make sure the bolt/carrier is WELL lubed.
have different mags been tried? what kind of mags? something quality like a pmag or good quality usgi, or random gun show mag?
-rvb