Non jacketed lead bullets may creep out of the casing in smaller frames. The crimping on the casing doesn't hold onto the lead as well as the FMJ, so with enough vibration and such from firing the gun, the last round may pull out a bit, which could cause problems.

Standard pressure lead rounds are fine. I've yet to have any issues firing lead +Ps through my 642.

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- The Federalist #46 -