I think I have read all of this discussion, and can't recall where anyone mentioned the effect of friction of the barrel on the cast lead bullet. The surface friction at speeds over 1000 is enough to melt the surface skin of the lead. Once the skin is melted, then gas jets can blow through.
The gas checks don't stop the melting of the lead skin due to friction. But they do scrape that melted lead along in front of it, and hydraulically the liquid skin of lead stays where it started, eventually solidifying.
The gas checks also seal the melted skin of lead from the gas jets otherwise seeking to blow past it.
Just my opinion, and I am sticking to it.