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Old May 8, 2001, 11:06 AM   #12
Cheapo
Senior Member
 
Join Date: November 19, 1998
Posts: 986
Johnny Guest, I wish I had my references handy, because I once knew from published pressure data (Alliant??) that max published loads for the same powder ran from 0% to 5% less in charge weight for lead bullet loads, as compared to their jacketed bullet loads for the same bullet weight.

True, some of this may be attributable to differences in bearing surface.

I've *heard* that at smokeless powder pressures, the accelleration of the lead slug causes continued obduration of the metal against the bore, resulting in greater a duration and magnitude of friction for lead bullets as compared to jacketed bullets. The initial resistance of jacketed is higher, as the rifling engraves the jacket, but once that operation is complete, the jacket resists the radial "slumping" of the lead and results in less overall friction and lower pressures, which in turn require a small increase in powder charge to reach the same velocity.

Maybe some metals expert engineering type can confirm this theory for us. Sounds plausible to me, especially since we need to keep velocities below 1500 fps for a certain 165-gr cast .30 rifle bullet because its pointy nose (no bore-riding forward shank) fails (slumps) under accelleration. Accuracy goes to pot.

Remember how Moly bullets take more powder to reach the same speed? Less friction = less pressure = less velocity, without a compensating increase in powder charge.

So....the general trend I have seen is that lead bullets *usually* take a bit less powder to reach max pressure and velocity than comparable-weight jacketed bullets. One illustrative exception, IIRC, was 125-gr bullets in .357 Magnum using WW-296. Remember, that's IIRC!
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