Dan - if you keep the speed down, you should be able to easily get away with a flat base, no gas check. I've not seen a comercial .50 cal. mould that has a gas check, althought I know there are custom moulds with them.
; The normal run of the mill mold like the RCBS heavy one has a very heavy base. I think this is a good thing, as it's generally more accurate (in my rifles) than the Lyman 520gr. Pick a powder that bulks up well, then use the foam or other 'filler's to create a 100% loading. This 'filler' will protect the bullet's base the same as a gas check would have - hense, gas checks are not needed. Cast and size the bullet .001" to .002" larger than the groove diameter and you'll not get leading. Use a good lube - SPG, Lyman BPGold, Lyman Moly, or mix your own - double boiler (one pot inside another pot of water) Beeswax/Vaseline in a 60:40 mix. The 60% beeswax seems to give a very good consistancy (viscosity) for both black pwoder and smokeless shooting. My .50 shot to 2,000fps with this home made lube, no leading. It gave 1 1/2" accuracy at 200 yards - tang sight peep and pin-head front sights off the bags using both smokeless and black powder - both runing around 1,500fps.

I suspect with reduced loads, you'll need a heavy bullet to help regulate the double. I might be 180 degrees off on that prediction.