Yes Terefang I think you're adding GURPS into Genesys in that the rules don't envision these scenarios. For example, if you look at RAW for assault rifles - it's not considering the pre/post 5.56mm NATO round and how the older 7.62mm rounds the FN-FAL carried meant less ammo and more weight etc. It's just not about that crunch, that technicality. It's about the story.
And the whole "SovBloc country but NATO ammo" - you do not need ammo types statted out individually to do that. Again, GURPSing the NaDS. You can achieve that same outcome through narration without saying "well, no, you don't have your specific type so... whoops." And it just needs you to run the story and tell the players that in this instance, a story point isn't enough to make ammo appear.
As for the Karabiner, raised by @gorthano - why have different types? The rifle has hardpoints. The book has scopes. Add the scope to the rifle hardpoint and //ding noise.