It's a little perplexing, but despite lower start pressures, some lead bullets have been observed to cause higher chamber pressures, in revolvers, in particular, than equal weight jacketed bullets. Seating depth difference due to shape difference may play a role, though some chambers are probably just filled more solidly by a bullet that bumps up. The limit of 4.0 grains strikes me as low, but should be tested. Certainly with jacketed bullets it doesn't contain as much energy as a military ball load. That would take slightly over 5.0 grains with this bullet.
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