A way to get this relatively early on in the game is to gain the sniper rifler on Liberty Island, do not purchase extra rounds of 30.06 ammo, and this should put you close to one or two rounds of sniper rifle ammo.
I gained this as a part of the Orbital Cannon achievement, which involves killing one of the two NSF terrorists on the north dock, from the top of the Statue of Liberty, just before talking with the terrorist commander.
Important note: you might want to save just before you head up to the top, when you hear the phrase "Easy..." on your communicator. Getting too close to the commander forces dialogue and after the dialogue, every terrorist still awake on the island is instantly killed.
As it's really challenging to kill one of the two NSF terrorists on the north dock from the top of the Statue of Liberty, you are going to have to save just before you take the shot. If you have more than one bullet, waste your ammo until you're down to a single round remaining, and then just take that single shot, reloading your save game if you miss.
Hitting one of the NSF terrorists on the North Dock will give you the Orbital Cannon achievement, but hitting them with only one round remaining in the chamber will cause this achievement to also pop.
I'm sure there are easier ways to kill someone with one round remaining in the chamber, but because you gain ammo for other weapons so fast, especially if you're looting bodies along the way, it's actually easier to just use the sniper rifle, as that will likely only have a few rounds in it by the time you're ready to shoot someone.
Conversely, if you don't care about the Orbital Cannon achievement, you could also just waste all of your sniper rifle ammo, bring it down to a single shot remaining, and shoot a random terrorist in the head, which is a "guaranteed kill shot."