Well, you can drive a lead bullet just real, real fast. And you can learn all about bore cleaning as being real, real rough!
Depending on the hardness of the lead, around 1,300 to 1,400 ft/sec or so is pushing the limit for "just lead". The use of a copper gas-check lets you get on up around 2,000 or 2,200 ft/sec without undue leading of the bore. These are approximate numbers, not exact, of course. There easily could be some tricks I don't know about.
I do know that 20 grains of 2400 in an '06, behind a 169-grain gas check gives you around 2,000 ft/sec. (It also works in a .308 and a .30-40 Krag, I learned last month. Works with 170-grain .30-30 jacketed bullets, too, in those cartridges. Use 18 grains in a .30-30 with a 100- or 110-grain bullet for a nice plinker.)
"Meddlin' around" on the benchrest, my .308 is sighted in for 2" high at 100 yards with normal 150-grain hunting loads. With the 170-grain plinkers, and the crosshairs "as usual", I was around 15" low at 100 yards--which meant I was hitting the ground in front of the target. Quite a spray of lead, copper and gravel on the target!
[This message has been edited by Art Eatman (edited August 15, 2000).]