Try a 6" 9x25 conversion barrel. I know they make them for the G20, but I'm not certain if they'll fit into the G29. Then again, why the extra money on all that stuff? Just buy a bunch of 10mm ammo and start blasting away at those little rats.
A different type of ammo isn't going to extend the sight radius on that gun, so just use 10mm loads.
Gen 3: G19,G20,G23,G26,G27,G29,G30,G30S,G32*,.50GI
Gen 2: G19
Gen 4: G17,G22
Gen 1: G17
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