I'm sure not going to shoot any $1 a loaded round 50 BMG ammo. Not in anything. I figured at least $5 a loaded round, and after the cost of a 50 BMG press and dies is amortized, and $2.50 - $3.50 a round for doing it myself.
Do a search, there are various 50 BMG resource sites on the Internet, they list the currently available rifles and actions. While there is an upper you can mount on an AR-15 lower, I suggest one of the purpose built single shot 50 BMG rifles. Many of the single shot 50 BMG rifles are in the $2K to $3K range. For $5K you can get a bolt action Barrett.
You will not buy an action only. No one I know of sells an action only. Be aware that a good 50 BMG barrel (an M2 surplus) will cost you over $200 at the very least. You need a lathe, a reamer, and several special and expensive gauges to properly install said barrel.
