There's two Nail Gun spawns on opposite ends of the map. When each match starts, you'll see an overview of the map and you should be able to identify the Nail Gun symbol. It's not the bow, nor is it the icon that looks like an assault rifle. It's usually found on something that looks like a workbench. The nailgun can be found on the following maps. It's worth noting that I'm basing this off of where they spawn in the Bounty Hunter gametype.
Backwoods: Probably the easiest map to obtain the Nail Gun. When the match starts, simply turn around and run towards the back of the map. If you go far enough, you'll see the weapon spawn near a small trailer or in a large wood cabin in what appears to be a lumber yard.
Black Friday: Spawns the weapon near the escalators on either side of the map. Keep an eye out, as one side is on the second floor and the other is on the ground level beside the escalator. This one is also near the spawn point on Blood Money, though you'll have less luck obtaining it as it's in the direction most players will be heading.
Code Blue: Has one spawn that is easy to identify an done that is kind of weird. The first is near the construction area. There's a series of "stairs" in the back of the building, which you use to get to the top. The two buildings connect by a thin beam, so be careful not to fall if you climb up the wrong building. The other spawn is towards the "city" area on the other side of the map. In a small, fenced off area beside a ladder you'll see the Nail Gun. It's a good idea to keep an eye out for the symbol, it can be tough to find the first time.
The Beat: My least favorite map to attempt these kills on. Usually I would just play this map out as I would normally for this reason. The Nail Gun spawns in a two floor building relatively close to the spawn. People often scoop them up on the way to the middle, hence why you don't see them very often.
Alright, now on to how to actually get kills. This is a close range weapon at it's core. You can get kills at medium range, but the damage per shot will be much lower. To make matters worse, when you aim down the "sight", it's really just looking down the front of the gun as there is no sight. I still used this from time to time, but the hip fire works just as well when you're up close. As much as it pains me to say this, your best bet is to camp it out near the spawn when you pick up the gun. With the limitations on range, you can't run and gun as well with this weapon. Also, it's a single shot weapon. So once you have a shot lined up, don't hold the trigger. You'll have to tap it rapidly if you hope to kill the enemy.
As for what class to use, I'd recommend either the Professional or Enforcer classes. With professional, you can set cameras down before you pick up the Nail Gun which makes it easy to know when enemies are in a close vicinity. This is key when you're trying to get the jump on them. As for the Enforcer class, this really just boils down to the fact you can equip the Ballistic Shield and have your back covered. This saved me a few times, allowing me to net some extra kills.
It's worth noting you should NOT try placing cameras after you pick the gun up. This causes you to drop the gun and you'll have to quickly find it and pick it back up or risk it disappearing. This is the same if you switch weapons, or try to use any equipment while the Nail Gun is in your hands. If an ally picks the gun up and dies, you have a short time to pick it up after as well. Though this is rather tricky.