I little trick I came up with years ago is to use “caulking backer rod” to fill some of the head space over the shot. Sometimes, the wad choice, hull, crimp or reloader not always cooperative and one ends up with a hole in the crimp. I use 5/8” (~16mm) diameter backer rod and cut it to about 0.20” (5mm) thickness. They fit perfectly in a 12-gauge hull and weigh practically nothing, about 3/10 of a grain. I use a long utility knife blade and they cut quickly and easily. After the shot drops, I just insert the cut piece over the shot at the crimp starter station. The foam is easily compressible and doesn’t affect the crimp, just acts perfectly as a shot sealer. They are so light weight; I’ve tested the shot pattern and as expected, the pattern is unaffected. When the shell is shot, sometimes you see the foam flutter back to earth a few yards out, no different than the other confetti on some shells. I have done quite a few over the years without issue and have been incredibly pleased. Use it as you wish but I have found it helpful when needed and not burdensome.