Any weapon not listed is NOT needed, as you skill with them cannot be upgraded. These include things such as the Rocket Launcher, Sniper Rifle, melee weapons and thrown weapons.
Method:
Gather / buy plenty of ammo for the weapon skill you're levelling
Park a vehicle, or several vehicles, in an owned garage
Allow the garage door to close
Step forwards until the door opens
Unleash on your vehicle(s) until it bursts into flames, but does NOT explode
Quickly step back and allow the garage door to close. This will repair the vehicle to its original state
Repeat until skill is maxed
Switch to next weapon and repeat from the third step above
Remember that weapons of the same type share the same slot, so (for example) you cannot carry a Pistol and a Silenced Pistol at the same time. If you're having trouble getting a particular weapon, the easiest way is to play more story missions until Ammunition begins to stock them.uch easier than, say, stealing MP5s from SWAT teams.