I just tried out some things with the MXC 6.5mm Assault rifle and figured out there's an animation glitch while changing the stance from Combat stance to fully standing.
While moving the stance, the player shortly pops into the high prone/sitting state animation before he finally reaches the highest stance.

It doesn't happen with pistols, but I wasn't able to check whether it's only for the MXC 6.5mm or also for other assault rifles.