In the itemgroup, add the “ammo-item” and “charges” entry.
{ "item": "m1911", "ammo-item": "45_acp", "charges": 7, "container-item": "holster" }
In this example, the m1911 will spawn loaded with 7 rounds of 45 ACP. It’s also in a holster, but that’s not relevant.
I believe that’s what you’re looking for. You can see other examples of this here and there. That specific example is from the professions.json.