All right, here are the specs of the smart guns and the pulse rifles per the Colonial Marines Technical Manual. Not sure what exactly is their source for this stuff, the acknowledgements credit a lot of folks not directly involved with the movie, while James Cameron just gets a creative nod, so this stuff could be coming from the authors head, not from JC's technical notes from ALIENS. IIRC, the DVD has a bunch of tech notes on it, I'll have to look this weekend.
Anyway, the M41A pulse rifle fires the US M309 10mmx24 mm round. It has a bullet weight of 210 grains at 840 meters per second (!!!) (that's 2757 fps, pushing 300 win mag territory!). It states there is some sort of recoil dampener. It has select fire, 4 round burst, or semi-auto (the author confuses select fire with semi-auto). The magazine holds 99 rounds, but is commonly downloaded to 95 to reduce jamming. The carry handle battery pack is good for 10K firings, and can be recharged by the rifle rack or a portable power supply (BIG shortcoming there). The bullets themselves are explosive, designed to penetrate armor then explode. The rifle is a pig though, weighing in at 4.9 kg (almost 11 pounds). ROF is 900 rpm.
The grenade launcher holds 4 rounds, and everything about the grenades themselves is correct from Slamfires post. My read is that the magazine holds four and you can have another in the chamber, but since I'm sure no one wants to walk around with a grenade at the ready, it is probably not SOP (any M203 guys out there who can weigh in on this?). They have HE, HEAP, a "bouncing betty" type, buckshot (yeah!!), baton, willy pete, and starshell.
The M56 smart gun fires a 10mmx28 M2500 round. The bullet is 230 grain and is "higher powered that the M309 round", suggesting an even higher MV. The explosive fuse can be operator set for instant detonation (soft targets) or delay like the M309. The ROF is 1200 rpm and the weight with harness is 17.82 kg (39 pounds). I can't find a capacity for the drum magazine, but the Leading Edge game (which correlates with the tech manual pretty well) states 192 rounds.
The tech manual has some quirks. For instance, they describe the Marines pistol has the M4A3, shooting a 126 grain 9mm bullet at a puny 282 m/s (around 900 fps). The associated schematic is of a nice 1911 though
They do show the VP-70 later on, but call it the VP-70, so I'm not sure if they are describing the same pistol (with the 1911 pic inserted by mistake) or if there are actually 2 pistols in use. The LE gamebook lists just one pistol, the VP-70 (though they don't give a model number).
As a side note, there are a few airsoft groups making M41As out of the thompson AEG and a body kit. At least one even has a working couunter! They are not cheap, but a hell of a lot cheaper than trying to build a REAL pulse rifle!
Damn, I AM a geek!