Maybe as a bit of a gun nut, I can lend a hand here.
The MA5C assault (technically automatic) rifle fires the armor-piercing version of the 7.62x51mm NATO cartridge, a round known to be hard-hitting, long-ranged, and, with the right barrel twist, very accurate. On the other hand, the BR55 and BR55 HBSR battle rifles fire the 9.5x40mm cartridge, which according to canon does have slightly more kinetic energy than the 7.62 NATO, but also has significantly more recoil which is exaggerated even further with the BR55's lighter weight. If you have ever fired a weapon on full auto or even burst settings, muzzle climb due to recoil is a huge problem. 7.62mm cartridges are notorious for that problem, and I imagine the the 9.5mm would be far worse. Thus, the MA5C is a lower-recoil weapon and thereby more accurate at range for extended periods of fire.
Against the M7 submachine gun, the AR has another advantage in its ammo. The M7's 5x23mm cartridge could be compared to the modern 5.7x28mm and 4.5mm H&K rounds, both of which have excellent penetration (even the best Kevlar body armor cannot stop the 5.7mm) and can shred an enemy at close range, they simply lack any stopping or knockdown power whatsoever. Therefore the AR wins this battle in ammo type.
The 32-round magazine of the MA5C is also not a disadvantage. Though its capacity is smaller than BR55 and M7, this should actually increase combat effectiveness. Even trained soldiers given a deep well of ammunition in their firearm will tend to go rock-and-roll and waste most of that ammo. Smaller magazines mean fewer rounds, thereby forcing the soldier to aim and think about the placement of his rounds.
Any other questions?
btw: The term is "magazine" not "clip" for the part of the gun that holds the ammo. Clips are actually a part within the magazine itself.
"Whoever said the pen is mightier than the sword never encountered automatic weapons."
-General Douglas MacArthur
The Pheonix War: Pheonix Rising:
http://www.halowars.com/forums/thread/378844.aspx