Basically, it would have a + or - (increase or decrease) in certain values of the gun (things like damage, spread, recoil, bullet drop and # of bullets coming out of the gun) but NOT things like fire rate or reload time).

E.g. My custom sniper rifle has a damage value of 15 and spread of 2. I want to give it a match-grade ammo type that makes it more powerful and accurate but more costly. The ammo would have stats as follows:
Damage +5
Spread -1