There is nothing, anywhere, that says that canon Heavy Pistols that do 10M are .44 caliber. And you'll be really hard pressed to find any gun-freak on this forum that'll second that it should be a .44. Especially when there's a world of difference between a .44 Special, a .44 (Auto) Magnum, and eg a .440 COR-BON Magnum.
But yeah, I think it's a fair call that to do 10M Damage, a gun in SR should be relatively as powerful as a .44 Magnum is today IRL. But that's just personal opinion.
Concealability isn't about ergonomy, it's about concealing the gun. To fit 15 rounds of something that looks like .44 Mag in a pistol, it would have to be really thick and high. A thick and high gun isn't very likely to be concealable, no matter how ergonomic. But, like I said, the Predator is fricken huge and still has Conc 5, so it doesn't matter much.
Going caseless wouldn't help, as far as I can figure. Looking at
real world caseless rounds, it looks quite likely that either such rounds would be slimmer and shorter by an extremely small margin (the space the actual casing takes, which isn't that much),
or they could be much shorter but significantly wider (especially when considering pistol cartridges -- with rifles you could save in length without adding width). And the main problem with having a large-capacity large-caliber magazine in a pistol is the diameter of the rounds, which would at best be reduced by a negligible amount.
If the books say absolutely nothing about it having a 10M damage code, a mag of 15, or anything else either, then just make up stuff for your games. The rules don't have to make any sense at all when you create such a gun for your games. Just don't post the stats on the net if you don't want me to rip into them.