The 50BMG heavyguns guns are around 67 inches long with a K-P muzzlebrake.I ship mine in a military surplus stinger missile container.
I think the guy selling them is still around? Ponderosa Sports is his web store if I remember correctly.
I just looked and he is sold out but here is his ad.I would maybe do a search to see if anybody else is selling them.
Rocket Box .50 Cal Rifle Case - Outside dimensions - 66" Long x 13" Wide x 13.5" High, Inside dimensions - 60" Long x 11" Wide x 13.0" High. Weight - 46# One of the best rifle cases I have found is a STINGER GUIDED MISSILE box. It is a US Military aluminum case with seal and a pressure release valve. Heavy latches close it. There are two heavy handles, one on each end. The corners are 1" radius. The color is olive drab. The case hinges open in two clam shells. It already has very high density foam inserts that can be cut to fit most rifles. Wheels & locks are easily added.
SOLD
OUT
27A
Rocket Box .50 Cal Rifle Case - Outside dimensions - 76" Long x 14.5" Wide x 15.5" High, Clear inside dimensions - 74" Long x 10.75" Wide x 14.5" High. Weight - 60# This is the biggest rifle case I have found. It is a LASER MISSILE box. It is a US Military aluminum case with seal and a pressure release valve. 8 Heavy latches close it. There are 4 heavy handles, one on each corner. The corners are square. The color is olive drab. The case opens into two clam shells. There are six internal, removable, rubber shock mounts. Wheels & locks are easily added.