The gas check maker I made was used to build Soda Can gas check for installation on 45 ACP boolits.
The gas checks would at first shear on the side wall as the bullet and gas check were pushed through an upside down Lee push thru sizer.
The problem was solved by lubricating the inside of the Lee size with some hard bullet lube.
The Lee sizer was warmed a little and the lube was smeared around the internal bore of the Lee sizer.
The lube made the gas check and bullet to be easily pushed thru the up side down sizer by a small hand press from Harbor Freight.
I may have made the cup height too tall for this application.
The cup wall of the gas check drags along the inside of the Lee push thru size and the friction must put enough force on the thin sode can aluminum to cause the thin aluminum to tear.
Thicker material with more strength will be tried the next time, the .006 thick roof flashing should be stronger than the soda can material.
After lubing the bore of the Lee push thru sizer the process work alright with the soda can gas checks.
The picture below is of the Harbor Freight Press, Gas Check Maker, Up Side Down Lee Push Thru Size, bullets with gas checks installed.