I'll have to double check what they used in the series and movies. I think they used similar weapons in Terminator Salvation, but that was at a prime rebel base that was heavily defended and even had access to helicopters. Conner used a similar weapon at the start of the movie too, but once again, he was part of a special strike force.
I don't think there is an issue with that weapon though, it'd make sense that our guys have at least some of the good stuff to defend themselves with.
Coming weekend I'll make a list of the kind of weapons and stuff we can use. Our resistance base is pretty much an outpost near the front, but it makes sense that Command would send (ir)regular supplies of weapons and ammo to these outposts. Since we're up against Terminators it'd also make sense that we have some pretty heavy hardware. Guns alone simply don't cut it.
In fact, I even thought about having some experimental EMP or electricity based weaponry to shortcut terminators, and I'm sure our inventor could create some EMP grenades or boobytraps as well to take care of those nasty buggers.