The purpose of gas checks is to allow a bit higher velocities than you would want without one--to avoid leading the barrel.
For instance, in a .30-'06, some 20 grains of 2400 behind the 169-grain lead gas-check bullet gives around 1,750 ft/sec. Negligible leading...Sharpe's book lists loads to 2,400.
I'd guess that unless the bullets you use are a very hard alloy, anything over some 1,400 to 1,500 ft/sec (?) would cause leading...