In straight cases, you should find that a base wad of automotive gasket material, or card stock (I like monopoly board at 1/10") will allow higher speeds than a non-base wad load when using plain base bullets. I have also used cup-down gas checks under plain base bullets with good results. In both the .458 and .50, I was able to load to 2,000fps with plain based bullets + base wad, without leading and with good accuracy. Note, the lead was hardened WW and .002" larger than the groove diameter to start with.