Some checks have such a tight fit that I can’t pop them on as I’m sizing (with my push-through sizer). This is usually because the GC shank is a little fat. In these cases, I apply the gas checks as a separate stage to sizing/crimping then on. I use a piece of leather tacked to a piece of hardwood as a base and a small brass or plastic hammer. This ensures I don’t deform my boolits tapping the checks on. I also lube all my GC boolits with 45-45-10, apply the GC then pan lube after the check is on. Saves a mess in my sizing die.