Ya.

It gets modded from the hunting rifle. You have to have the Gun Nut perk maxed out to have access to that .50 receiver though. You'll sometimes find them as weapon drops, but typically not until you're at a fairly high level.

After a while they are for sale from vendors.

I found one on a .50 Hunting Rifle in the low 30's level-wise. I stripped off the part and put it on the first of my Instigating Hunting Rifles I had found, converting it from a .308 to a .50. With the Instigating Legendary, it packs a huge punch on the first shot.

Note that if you find a Suppressed .308 you don't seem to be able to put the Suppressor on the .50 for some reason so at the moment I have a Legendary .308 with Suppressor and a .50 without.

there's a relatively easy one you can get

East, not West. And the location is on the opposite side of the map, so lower level characters will have trouble getting there. That area of the map is recommended for characters around level 25-30.

Yeah, sorry. East. I went there fairly early (probably low teens) and was able to complete the quest. I was sent to the settlement near there by Preston and then just wandered a bit more east and found it.

I think to change a 308 to a 50 is a level 3 perk.