Unverified. Very simple. Get a special forces machine gun, then go to the Butcher's room (meat processing under Seon's grocery) and unload 150 shots on one piece of hanging meat. You will get Perfect Gunner unlocked!

Submitted by PND

If you've got a machinegun when you're doing the Bomb Run, you can easily attain the Perfect Gunner achievement by unloading the weapon into Carlito's delivery van.

Park your vehicle (I used the van) right next to one of the bomb vans and you'll be somewhat out of Carlito's line of fire when he's throwing grenades. Stand between your van and the bomb van and you should be able to get off the entire magazine with no problems.

You can also get a machinegun early through a survivor. Trade any weapon for Brett's machinegun when rescuing the survivors from the Huntin' Shack. If you manage to hang on to it until you can do either of the above exploits, you get the achievement.

Submitted by Abiron