In terms of gameplay mechanics, when a weapon hits a wall, the game typically moves it to the side, causing it to stop firing (which is a questionable decision by itself). With other weapons, players receive feedback that the weapon angle is "non-functional," and the game adjusts the weapon to the side. However, machine guns do not pulled aside; they simply stop firing.
This can be very frustrating when a player tries to shoot from behind cover, everything looks fine visually, but the weapon doesn't fire.