I took this screenshot while scoped in with the XM-21, suppressed.
I found that if I scope in and don't brace it on something, the entire viewmodel freaks out and bounces up and down until it's, eventually, off my screen. Making it utterly impossible to hit anything. It happens other times even when braced, but it's not nearly as bad. It only jiggles a bit, yet, still impossible to hit anything.
I didn't find this issue with the other american rifle, haven't noticed it on the mosin either.