Is there anyway to obtain this gun without killing the NPC who owns it? Does reverse pick-pocketing still work?

I did it! you have to have no companions and I had accepted with NCR I hit with fist while he was sleeping then I shot his arm till I crippled it and he dropped it ( now I had logs of aggro and had to try this 3 times) then holster my weapon and use stealthboy and then picked up weapon plus I left area for 24 hours to make sure I didnt have any aggro, but I think I didnt need to do that last part.

I bought a silenced .22 gun and aimed for the head. as long as your hidden you won't get any reprocusions

He's only involved you one quest you can do as soon as you're there, after which you can C4 the [censored] out of him or kill him with a sneak attack.

However, I did get the gun by pickpocketing. I stole his ammo, punched him, he didn't unequip the weapon as I expected, so I slept for a few hours, came back, he had unequipped it.