Well, other than maybe in some video games, there never has been a .45 caliber MP-5. There might be ripoff of the MP-5 that uses .45, but not from H&K. There was, for a while, a .40 S&W version, which might be what you are thinking of.

And they work well, they just never really got a big market share. Which is a pity, because of all the SMGs made in the past 20 years, I've liked the on-paper potential of the MP-5/10 the best. Full pressure 10mm auto loads put it in the PDW class (and what it could do with sub-caliber pentrators in a light lead alloy bullet or a discarding sabot are VERY interesting), while the lower/FBI pressured loads match the performance of .45 ACP nicely. That versatility is why I love the 10mm cartridge. Nice, big bullets, that can pushed fast or slow as needed, good accuracy, and an ambidexterous safety (or in this case, trigger group) are a winner in my book.

I never heard anything about the .40 S&W version, other than a few law enforcement agencies adopting it in very limited numbers for thier tactical teams.

In terms of accuracy, at room range, the two are very similiar. But once you get out to 100m, there is a big difference. It isn't so much in group size as it is bullet drop. And while you can compensate for this with holding at a different point of aim, that kinda breaks down when the adrenalin is flowing and your buddy is screaming and and bleeding next to you.

And if you want to talk group size, don't try. The MP-5 does good, the mechanical potential of it's barrel, when fired from a machine rest with very good ammunition, is something like 1.5 to 2 minute of angle. The standard, military issue M-4/C-7 does that to, and the very good M-4-type rifles made by folks like Knights, Wilsons, and various other custom and semi-custom makers can beat that. You can get a 1 MOA capable M-4-type rifle for about the same cost as an MP-5 these days. At 10 meters, the difference doesn't mean didly, but at 100m, it can. That gives a good, well barreled M-4-type rifle with a decent trigger and good ammo much greater versatility than the MP-5.

One thing to keep in mind is that with expanding ammunition, 9mm still pentrates literally a dozen to a score of walls. 5.56 does about half of that. Or less, when you are using the super light, speed of thought projectiles I favor. THAT, and better range, not so much the ability to penetrate armour, is why people in law enforcement and hostage rescue are switching over. (The perp in body armour is a statistical fluke, and a nightmare that you use something with some legs against, or you just shoot them in the head. Thier faces are rarely armoured. In the North Hollywood shootout, it wasn't the lack of power on hand. A 12ga slug will beat as much or more armour as 5.56mm in most cases, but it doesn't have the range. LAPD just couldn't get close enough to amount to much until SWAT showed up.)

And yes, things like Glasers and Magsafes prevent overpenetration, but they are so expensive that they are just not feasible for training. And you train with ammunition you will use in the field when you are at that level, beucase there is no save button and there is no do overs. The only people who can justify to expense for these rounds as standard issue are air marshals, for obvious reasons. Also, they have a mixed rep when it comes to mechanical reliability- there are some teams that train specifically for dealing with hazardous enviroments where there are things that you just don't want to shoot who have revolvers specifically for using glaser-type rounds in those situations, becuase there is no failure to feed with a wheelgun.

Jim, don't take this the wrong way, but when I hear people saying they think the MP-5 is pretty, they usually are counterstrike players. You wouldn't by chance be one, are you? Pretty is in the eye of the beholder, which is why I think the Thompson is prettier than the MP-5. But I also say kids these days have no style. :P Nor do thier parents. There as been a distinct lack of classiness on this planet since the early 60s. :P