Hey
Im quite a weapons expert, so if you want to know anything just ask.
In Nemisis and Small Victories. The USAS (has different names but the usual is Universal Semi-Automatic Shotgun) was used. The USAS is powerful and does not require the cartridge to be 'pumped' into the firing chamber like Pump-Action shot guns use.
SG-1 started with the use of the Heckler & Koch MP5-A5, quickly moving onto the more tactically equipped Heckler & Koch MP5 RAS.
Finally moving onto the Fabrique National P90. The ultimate SMG in my opinion. Hopefully we will continue to see this amazing weapon being used....