There were two runs of the 4" Ti-cylindered .40 S&W 646 - the tapered lug/slab-sided barrel 36 oz PC Shop one from 2000 - MSRP ~$850 and the special production regular barrel/full lug 38 oz one from 2003 ~$560 MSRP. I broke down nearly eight years back and bought my 296 ($349 on closeout) and 696 ($439 regular price) new on the same day - got another $10 off for buying two new S&W's. The PC 646 was $729 there, I believe - the last new production 646 I saw was $449! Those were the days... and that wasn't that long ago. During the ammo shortage, everyone had .40 S&W.
A 10mm would fit the 646's .40 S&W chambers? In O.J.'s "If it fits..." scenario, as I found when I removed my first S&W's , a .45 Colt 625MG, cylinder to clean it, some .454 Casull ammo would fit in that same cylinder's chambers - and be below the cylinder exit's face. That was a shock. That and the 10mm in a 646's chambers, I feel, is a very bad idea. I would never do it. I soon sold my SRH and said goodbye to the .454 ammo & reloads I had, determined that SAAMI spec .45 Colt was 'enough' for me.
Never liked the 10mm - or saw the need in an N-frame .41M - the .44 is so wide-ranging, Russians - Specials - Magnums, and widely available - try to find .41M's.
Stainz