As some of the more gun-savvy players here might have noticed, neither the MAC-10 nor the Fang 45 share ammo with the .45-caliber sidearms in-game. In reality, the MAC-10 is chambered in .45, and as for the Fang 45, it's obviously chambered in .45 (because of its name). Instead, in-game the MAC-10 is coded to use 9mm (Like the MP5, Mini-Uzi, and M9) and the Fang 45 is coded to have its own ammo. I deduce that this is probably a developer oversight and they thought that players wouldn't notice.
I understand that Call of Duty isn't the most realistic game, but I believe that these weapons should be coded to use the correct ammo (.45 ACP, like the M1911 and USP .45), not only because it would be correct, but it would also give players more options to use certain weapons because they would be able to share ammo with other weapons already in the game. This would allow players to pick up ammo from weapons on the ground, or use a .45 SMG and .45 sidearm as an alternative to running the Bandolier perk, which is one of the reasons some of my Create-a-Class loadouts have a 9mm SMG with the M9 as a sidearm.
Also, this quirk isn't just for the SMG's, but also for other DLC weapons like the XM-LAR and the PKM, which aren't coded to use their respective calibers. I know that ammo compatibility should be possible with DLC weapons, because the Rangers and the Kamchatka-12 share ammo with the other shotguns.
Please developers, release a patch or update that will alleviate this issue. This should be an easy fix.