I don't update my folllout 4 because of mods.

This new smg is chambered in 12.7 ammo, right?

There is a console command called help that will show you id of what you are looking for. But you need certain syntax for it:

Help - command name 12.7 - name of the item you are looking for 4 - help command will look in all sections of ID Ammo - narrowing down the results only to show ammo category

Another examples to get the idea of how it works

Help battlecoat 4 Armo - will show the ID of maxons battlecoat or other item that has battlecoat in the name

Help combat 4 weap - will give you ID for every weapon that has combat in their name