If your groove diameter is .429 and you size your bullets .430 but have .433 throats, you’re probably going to have leading. The reason the +.001 thing usually works is that properly sized throats are .001-.002 over groove diameter and most S&Ws held to those specs. M25-2s were notorious for having .451 groove diameters and .454-.455 throats. I didn’t know about the 29-2s but apparently that’s the case. Elmer Keith didn’t have to worry about this because any M29 S&W sent him would’ve been gone over with a fine-tooth comb to ensure it was within specs for use with cast bullets.

Back in those days, Smith could sell every M29 they made due to the Dirty Harry movies. Most of those guns, if they were shot at all, were shot with jacketed factory ammo, so leading wasn’t an issue. If you raised the leading as a warranty issue, chances were good that S&W would claim that the warranty was voided by the use of non-factory ammunition.