If you get a mold for a bullet that uses gas checks you have a choice; You can load the bullets either with, or without the gas check. Mid-range loads up to around 1000 fps can be shot without a check, but magnum loads will really benefit from wearing a gas check. Soft or hard alloys, lubed or powder coated, checked or plain base, these are all choices that you get to make when you cast your own. You don't necessarily have to use a gas check, (though bullets designed for them generally shoot batter with them installed), but when you choose a plain based design (especially in a magnum caliber), it "might" limit your options. (I've shot powder coated plain based SWC's in my 44's at magnum velocities, but I've had to cast them with hard enough alloy that it limits expansion)