Compatible plug.
As regards bullets, while the calibre could be exactly the same, the warhead might vary in effect; however, in our case, what matters is the power of the propellant, since guns are usually optimized around that: slow burning, fast burning, extra large.
So the military tries to standardize bullets and magazines, so that any branch of the military, or militaries, can fit that into the firearms that they have, which also tends to work out for civilian customers.
The Confederation is actively assisting building up local Home Guard and planetary defence militias, so they would have some influence in the standards adopted, and economies of scale probably would tip that to default designs, especially since they ant to integrate in greater or lesser extent with the Army and Navy.