I think the problem is not in the gun itself, its in the terrain under the gun. People have complained numerous times that the effect we know of as "water" extends into the green stuff, and the effect we know of as "land" can extend into the blue sometimes. The reason the cannon rounds take out ack so much more easily is becuase of the explosion. Proximity is enough to take out an ack gun. With MGs you have to actually hit it, so accuracy is important. Well, if the "ground" acts like "water", there is no proximity explosion. Therefore, no matter how big the bullet is, you have to hit it to do damage.
This is just a theory, but based on observations and other people's gripes. I could be wrong. Wouldnt be the first time.