S&W has already tried the medium frame .40 revolver concept before with the 646 and it didn't catch on (was only made for about a year IIRC). The 646 was made on the L-Frame most likely because the J and K-Frames simply aren't big enough to shoehorn a .40 into. You see, the K-Frame already requires the bottom of its forcing cone to be ground flat in order to clear the yoke. This makes the forcing cone thin at the six o'clock position and thus is the root of the K-Frame Magnums' issues with lightweight .357 Magnum ammunition. A .40 caliber barrel would have an even thinner forcing cone which would only exacerbate such issues. Similarly, while J-Frames do not have the "flat spot", their forcing cones are not very thick to begin with and there simply isn't enough metal there for anything bigger than a .38/.357.
Even in the L-Frame, .40 S&W is pushing the limits. While L-Frames are perfectly up to .357 Magnum pressures due to the thickness of their forcing cones, S&W has stated before that the L-Frame would not hold up to .41 Magnum (which is only 1,000psi higher max pressure than .357 Magnum or .40 S&W). I suspect that this is because of the larger diameter case and bullets necessitating a thinner forcing cone and chamber walls.
Now, the new Charter Arms Pit Bull is fairly similar in size to a K-Frame, but it's relatively new on the market so its durability has yet to be seen.
Part of the issue as well is that one can pretty easily match or exceed the ballistics of the .40 S&W with the .357 Magnum though the initial bullet diameter is smaller (with careful load selection, the expanded diamters are very close). .357 Magnum is already a well-established revolver cartridge and most so-chambered revolvers still on the market had their bugs worked out years ago. Likewise, if bullet diameter is what you're after, five-shot .44 Special revolvers like the Charter Arms Bulldog, Taurus 445, Rossi 720, and S&W 696 are available in the same size and weight package as a .40 and have an even larger diameter bullet with ballistics fairly comparable to a .45 ACP.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar
Last edited by Webleymkv; September 7, 2012 at 08:10 AM.