IMO you can do better.
I think the novelty of a 50BMG going bang will soon fade an you will be stuck with clumsy inaccurate weapon.
If were going to build one if these ( im not)
Id use a 16 pitch thread what ever size the blank OD is.
Mosburg sells chambered blanls for about $175.
A standard cal rifle with a take off barrel could likely be built for about $ 45