"Ammunition from pistols wouldn't work in SMGs.": Yes they do. You can take your .45 ACP ammunition right out of your Colt 1911, put it straight into a UMP .45, and the UMP will fire it fine.
I'm curious. What is it that your hoping to accomplish here?
What sort of game mechanic is it that your hoping to change.
Are you trying to have 3 different pistols, along with an MP & SMG on you that all fire the same ammo?
Given most of them use clips, I'm not sure what sort of space savings your getting.
Cost is already the same.
Frankly, as a player, I'm glad that someones SMG no matter the brand, has ammo that all works for me. That way even if I'm not interested in salvaging guns & having to change over ownership, I can always just take some spare clips off the bodies & use that ammo in my own clips later.
If not for that extra -1 AP every heavy pistol gets, you could easily wrap up every gun from Hold Out through SMG with 4 calibers, but I don't know if I want there to be 3 calibers (maybe .40 S&W, .45 ACP, and .357) to be specific for heavy pistols. The solution to that problem could be just giving some SMGs some AP. At least that way the heavier caliber pistol rounds would be carried across platforms.
And why would there be only 4 calibers?
I mean right now just off really common rounds, I can think of a lot more then that to fill that many weapon groups.
.22LR, .22Short, .25ACP, .380Auto, .38Special, 9MM, 10MM, .40SW, .45ACP, .45Long Colt, .44Magnum, .357 Magnum.
And that isn't even touching what I'd call "exotics" like .454 Casull , .41 Magnum, 9mm Luger, .50AE. Etc etc.
But realistically I think that -1AP is there to represent the fact that you see more Handguns loaded in Magnum calibers than you ever would SMG/MP.
I don't personally see the worth in this, but that being said I think machine guns could be boiled down to a mere three calibers; 5.56mm NATO (.223), 7.62mm NATO (.308), and 12.7mm NATO (.50).
Personally I'm not a fan of .50BMG being anywhere on the MG list as that minor DV boost isn't at all accurate compared to what a .50 does v/s a 5.56 or 7.62. It should be reserved for DV's in the 14+ range IMHO.
But, that nitpick aside, if you needed a caliber for DV11, I'd go old school 30-06 BAR just for the MG list, but really, I'd do something like below.
For the sake of a complete list, I thought I'd see what I could come up with.
6DV - 22LR
7DV - .380
8DV - 9MM
9DV - .45ACP or 5.56MM or .410-Gauge (Depending on frame)
10DV - 6.8MM or 20-Gauge
11DV - 7.62*39 or 16-Gauge
12DV - 7.62*51 or 12-Gauge
13DV - .30.06 or 10-Gauge
14DV - .50BMG
Not that I call this list a good one, over all, I feel the DV's are really too limited, but in the end, its a game so I don't care.
But if I had to label a DV, I'd go with the above as a good starting point.