The Traveller weapons listed can really be thought of as "standard" weapons. There would have to be a lot of variation in stats in reality.
Far example, The original AK-47 and the M-16 are roughly analogous, and would both be "assault rifles" in Traveller. Both have roughly the same muzzle energy from their standard military rounds. However, the (original) M-16 had a 20 round magazine, while the AK has 30 rounds. The M-16 also weighs only about 2/3 as much as an AK. Now the M-16 has a standard 30 round magazine, without changing anything in the functionality of the actual weapon.
I think what you're looking fo is easy to make. Use the stats for the standard Gauss Rifle, but increase the ammo to 100, 200, 250, or whatever. Add the weight of the rounds, plus a little extra for a bigger battery pack, into the magazine. Do away with the fancy electronic thermal combat sight in favor of a more simple low-light, low magnification combat sight to reflect its new role as essentially a high rate-of-fire bullet hose.
A Gauss weapon would probably never be "belt fed", do to the lack of cartridge cases and possible feed issues. Instead, it would likely be a big box magazine (maybe the size of a big dictionary). It could also be a track-style linkless feed like the GE miniguns use.
Anyway, this is how I view Traveller weapons. Not every SMG or ACR you find will be the same in ammo capacity, sights, weight, or any other single technical aspect. The galaxy's a big place!