I shot a friend's Al/Sc/Ti 340 years ago - unbeknown to me to be loaded with 'hot' .357M CorBon ammo. Simply put - nothing to hold on to, grip wise, no mass - a literally horrible experience. It made an 8+" .500 Magnum, my 7.5" .454 SRH, a .45-70 BFR, and even my arch-nemesis, a .50 AE DEII mild in recoil by comparison (The .500 Mag & BFR weighed so much my arms grew tired of holding them up! The DEII wasn't a bad straight back recoiler - it was the rotational 'jerk' as the bolt stopped that hurt! My .454 SRH put the recoil impulse straight down the arm - great ergonomics!).
My only J-frame .357 Magnum, a 3" 60 Pro, like every other S&W .357M I've owned, carries the Remington R38S12 .38 Spcl +P 158gr LHPSWC for SD - just like my .38's - including my still 24/7 protector - my 642. I do carry 'Magnums' in two J-frames: 115gr Speer Gold Dots in .327 Federal Magnums in my snappy recoiling 6-shooter 632 Pro and Hornady 45gr FTX 'Critical Defense' .22 WMR rounds in my 7-shooter 351PD.
Weight savings over the same .38's in a 642 vs the 340's is not negligible. But, empty weight - the 351 PD weighs 4.2 oz less than the 642 - add 1.1 oz for seven FTX rounds - add 2.7 oz for five .38 +P. Loaded, the 351PD then weighs 11.9oz; the 340PD 14.1 oz; the M&P340 16.0 oz; and the 642 17.7 oz - all loaded. Not that much weight savings of the M&P340 compared to the 642 - just the added ability to launch .357M's - a dubious proposition. If you want a wide palate, besides it's designed for .327 Federal Magnum rating, the 632 Pro will shoot .32 H&R Magnums, .32 S&W Long, .32 S&W, and .32 ACP (There is a vestige of a rim present!), making it a .32 omnivore of sorts.
I may have added the 60 Pro, 632 Pro, and even the 351PD as possible EDC CCW's, but my venerable 642 still gets the nod here for 24/7 CC. May just be a tad heftier - but familiarity and proven dependability trump novelty and lite weight. Of course, any rationale that moves another revolver up the 'drool' list is valid...
Stainz