A gas check is normally a copper or aluminum gasket type cap that is installed on the bottom of your cast bullet.
First, check to see If your bullet allows for the use of a gas check. Molds that can accept a gas check have a rebated rim on the bottom of the bullet. LEE molds will tell you if they can accept a gas check by including a "C" at the beginning of the mold description. For example, see mold 90451. The mold description states the mold is a C255-55-RF. This means that this mold can accept a gas check (C), has a diameter of .255", an approximate weight of 55 grains and has a Round Flat (RF) nose. If you aren't sure if your bullet or mold will accept a gas check, check with the manufacturer.
Second, if your mold can accept a gas check, obtain the correct size for the bullet. Gas checks are available by caliber range - the above mentioned mold would use a 22 caliber gas check. A .308 diameter mold would use a 30 caliber gas check, etc.
You do not need to use a gas check but you may want to. Gas checks help prevent leading and also create a seal between the bullet and the barrel to withstand pressure and heat. By increasing the pressure behind the bullet, it then increases accuracy.
To crimp gas checks onto your bullets with the Lee Bullet Sizing Kits, first lubricate the bullets with Liquid Alox, push a gas check on the base of the bullet and set the bullet on the punch. When the bullet is pushed through the sizing die, the bullet will be sized and the gas check will be crimped on.
To crimp gas checks onto your bullets on the APP Press using the Lee Breech Lock Bullet Sizing Kit and appropriate Lee Sizer & Punch Kit, first lubricate the bullets with Liquid Alox. Before the bullets move from the feed tube to the Sizing Die and Punch, push a gas check on the base of the bullet. When the bullet is dropped into and pushed through the Sizing Die, the bullet will be sized and the gas check will be crimped on.
