As others have said, I understand the issue with lead bullets to be two-fold. The hexagonal rifling of the Glocks and other pistol-types does not get a good purchase on the bullet. This results in reduced accuracy.
However, it also results in lead getting stripped from the bullet by the rifle ridges. This lead can build up and could cause all the pitfalls of a very dirty barrel. Mostly again, a loss of accuracy.
That is how it was explained to me.
I suppose if it built up very heavily, then perhaps you could get a pressure spike if the barrel was marginally occluded by the build up.... dunno for sure
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