Why no supressed .40s?
 

I see lots of 9mil and .45 guns supressed on forums, and threaded barrels in both calibers. Dont see any 40s though. And it seems threaded 40 barrels are hard to find, which kinda sucks since compensators also go on threded barrels.

Is there any reason to use a pistol cartridge other than the .40 S&W?

Yes, the 40 isnt the end all, 9, 40, 45, 357....its all what you shoot well in the package. Im just wondering why i never see one with supressor.

What about the Cobra M2?

I think the Glock .40s are the only guns I've ever seen commonly fitted with suppressors. I don't know why the aren't used more for suppressed usage, perhaps they're just not as popular with special forces and stealth operators, where it's main users of .40s are FBI (who don't need suppressors) and movies, who don't use .40s as much as other calibers.