As a new member here let me just throw something out for consideration.For exemple,rubber is a viscoelastic material having both the properties of steel (elastic) and liquid (non-compressable).You can change its shape and its dimension in direction but it will not compress.I believe that lead does not exibit an elastic property nor is it compressable.However,subjected to sufficient force its shape can be changed and therefore its dimension.If that is true then lead bullets under certain conditions will bump up or obturate.Consider firing a revolver or rifle bullet that is undersize for the bore.A power charge that is quite light for caliber will generally result in leading because there insufficient pressure to bump up the bullets diameter to fit the bore.Now increase that powder charge up towards maximum and with that being the only change it should be expected that leading will be eliminated.Why,most likely because the bullet bumped up to bore diameter sealing the bore,stopping gas blowby and holding the rifling unlike the 1st issue.
Whew