I would like to refinish my 50 BMG, but it is unfortunately far too large for my cabinet and it can't really be disassembled realistically (shell holder bullpup gun where the barrel is about 90% of the length).

I bought some air cure cerakote (since it similarly won't fit in my oven), but what is the best way to prep this for coating? The current finish is parkerized, so I will need to degrease it extremely thoroughly, but from what I read parking actually holds on to cerakote pretty well.

I've done a few small jobs but never done a gun this large, so any hints would he appreciated.

My buddy and I had some parts too long for the cabinet once, long barrel or something.
We cut a circular hole in one side of the cabinet, six inches or so,  mounted a PVC pipe flange to it, and screwed on a section of PVC pipe with the end capped. When we didn't need the pipe section, it was removed and the cap screwed directly to the flange.

The part that didn't fit in the cabinet could be loaded in through the pipe, and then the cap placed on.

Sand it real good with fine grit sandpaper. Has worked fine for me.

This is a good idea but I think the weight and shape of the gun in question might make it difficult.

I'll go the sandpaper route, and ghettofab my cabinet if I don't get good coverage.

IIRC, Cerakote is pretty adamant about a blasted finish, but anodizing can be just lightly blasted. Duracoat is far more forgiving, can be air-cured, works well with Parkerizing, and is still pretty durable. I would definitely come up with a way to give everything a long soak in acetone before going either direction though.

I’ve used a portable sandblaster and blasted the parts on a big tarp in my yard a few times.

^^This

When I'm doing motorcycle or engine parts, I'll take them outside the shop and run my air hose to a gravity feed sandblasting gun.
Just remember to wear eye pro, a face shield, gloves, and ideally cover your entire body or you'll be digging blast media out of your ears for the next week :)

Doesn't need to be anything fancy: