.303 British Service (The round for the Short-Magazine-Lee-Enfield) is 7.7X56mm (Rimmed). The M1 Garand uses the .30-06 Springfield, which is 7.62x63mm. The AK-47/AKM round is 7.62mm Soviet, 7.62x39mm. The 5.56mm NATO is 5.56x45mm while the 7.62mm NATO is 7.62x51mm.
Now, things get confusing as the differences in brass and powder used at different times gives different ballistics, as well as age and denaturing of old ammunition.
Finally, those are all Cased (Brass) ammunition. Shadowrun has Caseless as the standard for at least military firearms, which means there'd be new calibers, new changes in powder charge, and so on. A lack of named calibers means that we can't make any decisions based on the new firearms at all. Older ones, a bit more, but even then... What kind of quality of ammo is available in the 2050s-2070s? Old stuff that's getting older? New stuff that's made on the cheap? Handloaded brass from small cottage industries (Like the Reserve that makes ammo for the Sons of Anarchy amongst other things)?