Ulrich,
I am puzzled by the results of your accuracy testing.

Since you have experienced the same results with 0.429" and 0.431" diameter bullets I believe that the chamber throats are not an issue.

I have a number of S&W revolvers with varying lengths of SA trigger overtravel and they have not adversely impacted my ability to shoot them. However that is subject to individual variation.

I use a 245 gr. LSWC 0.429" with 4.3 gr. Trail Boss powder in my S&W 696or 624 and have not expereinced any accuracy issues. My lead bullets are not ultra hard.
Tony