I, personally, would use Cyberpunk 2020. It's very simple to learn, and converting guns is extremely simple. Here would be some example:
Glock 26 (WA 0, RNG 30, ROF 2, AMM 9, DMG 2d6+1, CON P, REL ST)
Glock 19 (WA 0, RNG 50, ROF 2, AMM 15, DMG 2d6+1, CON J, REL ST)
Glock 17 (WA +1, RNG 50, ROF 2, AMM 17, DMG 2d6+1, CON J, REL ST)
Glock 22 (WA +1, RNG 50, ROF 2, AMM 15, DMG 2d6+1, CON J, REL ST)
Glock 20 (WA +1, RNG 50, ROF 2, AMM 15, DMG 3d6+1, CON J, REL ST)
Colt M1911A1 (WA +1, RNG 50, ROF 2, AMM 7, DMG 2d6+3, CON J, REL ST)
H&K MP5A2 (WA +1, RNG 100, ROF 2/3/20, AMM 30, DMG 2d6+1, CON n/a)
H&K MP5A3 (WA 0, RNG 100, ROF 2/3/20, AMM 30, DMG 2d6+1, CON n/a or L)
Uzi (WA -1, RNG 100, ROF 2/30, AMM 32, DMG 2d6+1, CON L, REL VR)
Colt M4A1 (WA +1, RNG 300, ROF 2/25, AMM 30, DMG 5d6, CON L, REL VR)
Galil ACE 5.56 NATO (WA 0, RNG 300, ROF 2/25, AMM 30, DMG 5d6, CON L, REL VR)
FN FAL (WA 0, RNG 400, ROF 2/20, AMM 20, DMG 6d6+2 CON n/a, REL ST)
AK-47 or AKM (WA 0, RNG 200, ROF 2/20, AMM 30, DMG 5d6+1, CON n/a, REL VR)
AKMS or PM Md. 63 (WA -1, RNG 200, ROF 2/20, AMM 30, DMG 5d6+1, CON n/a or L, REL VR)
SIG 550 (WA +2, RNG 300, ROF 2/3/20, AMM 30, DMG 5d6, CON n/a or L, REL VR)
Dragunov SVD or SR-25 (WA +3, RNG 500, ROF 2, AMM 10, DMG 6d6+2, CON n/a, REL VR)
H&K PSG-1 (WA +5, RNG 600, ROF 2, AMM 10, DMG 6d6+2, CON n/a, REL ST)
Remington M40 (WA +5, RNG 600, ROF 1, AMM 5, DMG 6d6+2, CON n/a, REL VR)
Barrett M82A1 (WA +4, RNG 1000, ROF 1, AMM 10, DMG 6d10, CON n/a, REL VR)
Additional stats like recoil or encumbrance could be given to increase the difference between calibers and firearm sizes.