Choose from one of the 2 options

Use OpenIV or SparkIV to place the contents of your option here:
GTAIV\pc\models\cdimages\weapons.img\

If you want the gun to use 6 bullets but have increased damage, place WeaponInfo.xml in GTAIV\common\data

MAKE SURE YOU BACKUP YOUR VANILLA FILES AS YOU CAN'T UNDO THE MOD ONCE YOU'VE REPLACED IT