Comment on Object FFE3A2 by Hoborginc
I'd agree with the established cartridge, and 7.62x25 is a very good choice. It needs to make a comeback.
Now, provided the barrel is built to the right tolerances, I don't see why you'd have to change it. Seems the headspace would be the problem.
Unfortunately, the steel-cased ammunition is a recipe for disaster. The French have chronic issues with ammunition compatibility for their FAMAS, and they're looking for a replacement now. It really defeats the purpose of standardized ammunition. So, if the ammo has to be steel-cased, then proprietary rounds would be essential.
The best thing to do in all likelihood is just have an upper receiver that's modular.