The MA5K and M7 would make the most sense for the Pillar of Autumn, the level being a confined space with many close-quarters battles. It would be even better if you started off the next level with the MA5K for consistency (since you jut came off of the ship, and it is unlikely that you magically receiver an MA5C). Maybe have an MA5C lying on the ground from one of the Marines.

Also, do the MA5C and MA5K utilize the same ammunition (if they appear on the same map together)?