To add, dirty comes down to what powder in play for the ammo, and don't confuse that with bolt unlocking at the correct timing instead.
Also, the shorter the barrel, less time for full burn of powder before bullet passes gas port for bolt to start unlocking, so on short barrels, dirty action is more of the norm, even if cleaning burning powder is used in the load. Add a suppressor into the works, that does have higher back pressure, and want what the suppressor baffles collected from the shot, is going to end up back in the action from the back blast of the suppressor back into the action one the bolt/spend case is out of the way.