Each gun can be activated anywhere at any time, simply by equipping an/or shooting. If you play the game for long enough, you're sure to pick this achievement up by accident, but the VR Arsenal will allow you to do this in minutes. The seven guns you need to try are as follows:
MK-I EMR 7.62mm
SPMG .388
MAR-10 .338
Mk14 7.62mm
NAVID 9.3mm
CYRUS 9.3mm
ASP-1 KIR