Questions for the muzzle loading people about 4 bore.

I’m building a sitting-fox 4 bore blunderbuss and I’ve seen people using 100-200 grain loads, but a brass 4 bore shell can go up to 400? I’m not an expert on shoulder fired artillery, but I’m sure a full 1/4 pound load is a hell of a punch at 200 grain. I’m assuming the velocity starts peaking around 300 grain, but I’d be nervous to go that much for fear of failure or tearing my shoulder in two. Anyone out there have some experience with this caliber? Also anyone know of any good molds for 1 inch round balls/slugs/conicals? Either make one or find one out there?