I think the short end is designed for a gas check. I'm not getting a ton of lead, just a bit of spray above the forcing cone. The large end will fit into the barrel, the tight end accepts a sizing down to .451, which is where the gas check bevel is located. This will be the winter challenge this year. I just got out of the shop where I cast up sixty in my hardcast mixture of 50/50 lead/linotype. They formed well and turned nice and frosty upon cooling. The mold is marked .451 220...I don't know how he got it down to 220, I get readings all the way to 260 with 1:20, 254 with HC. For an aluminum mold it throws a nice bullet, ordinarily I'm not a fan of aluminum...I get surprised from time to time. I was never a big fan of brass until I got my hands on an NOE mold out of Australia, terrific mold. I have a few aluminum molds that throw nice balls for my muzzle loaders on from Moose Molds is amazing, he's no longer in business. It seems the newer Lee molds are holding up better than before, more secure locking pins.