The caliburs have gotten bigger, though, judging by Hallfight. Somewhere around 80mm, maybe larger. The guns used made relatively little noise, a sort of humming, and had obvious power cables. They were firing subsonic projectiles, otherwise they would have been considerably louder. That partly explains the large calibur, since you need a bigger projectile to do more damage at a slower velocity.
The evidence points to some kind of magnetically propelled projectile. On the other hand, they produced a considerable amount muzzle flash as well, which is something a coilgun, railgun, or Gauss rifle would not do.
Note that the marine squad commander in Hallfight commented on the lack of gravity; they expected gravity when they went aboard and found none, so those weapons were perfectly usable in standard gravity, or even in higher then standard gravity (they couldn't know if the Shivans were used to higher-gee environments then Terrans and Vasudans). I suspect a simple form of powered exoskeleton is built into those suits. As I stated before, the only reasons they might have restricted their choice of weaponry (especially since they had no clue what they were up against) would be for fear of causing severe damage to the ship's internal systems, punching holes in the hull and causing an explosive decompression, or possibly the fear that the Shivans evolved in a higher-gee environment and standard weaponry would be too heavy.
Also, noting the Vasudan battle gear seen in FS1, I suspect they actually had better, more advanced technology for their ground troops: the Vasudans appear to wear a form of powered armor. It has power cables sticking out, so... It also appears to cover more of the body and be somewhat thicker then the Terran armor. This might also mean that Vasudan boarding teams (I think there's an Isis transport stuck on the front end of the Azreal you seen in the opening shot of Hallfight, but the picture is too dark to be sure.) would have done considerably better then the hapless Terran squad we saw in Hallfight.
Presumably, by the time of FS2, Terran troopers now benefit from Vasudan-type power armor as well.