Silly question. How do you gas check lead?
Not a silly question at all, if you don't know about old time reloading...
waay back in the dark ages, when reloading starting to take off, lots of things were tried, some worked, some didn't. One of the things that did work was placing a copper cup over the base of a lead bullet. This allowed a lead bullet to be driven very fast (to as much as 2200fps or so, in rifles) without serious leading.
The copper cup pervented (checked) the powder gas from melting the base of the bullet. There were many designs made with a "rebated" base on the bullet to take the "gas check", which was added during the lubing& sizing process for the cast bullets.
Lyman has an extensive line of bullet molds, with several gas check designs.
I don't think gas check bullets are as popular as the once were, due to the huge numbers of jacketed bullets available these days, but they are still sold, and used by a fair number of dedicated reloaders.