First you have to cast or purchase hard lead bullets a little above intended size which are designed for gas checks. The Lyman catalog will have a list. The gas check is manually placed over the base of the bullet, it is inserted base down into the sizer, and with one downstroke the gas check is seated and the bullet sized and lubricated. I have an old Lyman sizer that works great. As to when a gas check must be used, look at your bore after firing. One or two swipes with a patch and solvent will clear away the dirty powder debris. Inspect the bore especially the area closest to the chamber for signs of roughness due to lead buildup. If this is present either the alloy is too soft or you need gas checks. If you provide the cartridge info, maybe someone can tell you exactly what they are doing. Part of the art of sizing & using gas checks is getting the best sizing diameter for your particular firearm.
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