A .303 case might hold 50gr. to 55gr. at max with a bullet seated over black powder that is compressed 1/10", which is normal. This compression is done with a plunger, to the bullet.

You would not NEED a wad, perhaps, but any .303 cast bullet I know of, does not hold enough black powder lube to be able to keep the fouling soft. They have only 2 grooves normally, rounded and of smokeless powder design. BP lube grooves are multiple, wide and square to hold more lube.

On top of the powder, you would likely NEED a lube wad due to this, which will cut back on your powder capacity even further.

I am not sure you WANT to try compressing black powder. I would attempt it.

People who over compress black powder in BP ctgs. usually are not any where near the winner's circle as overly compressed black powder charges do not shoot well with CAST bullets. Jacketed bullets are something else.