It's a really cool idea having your gun affected by dice rolls like this, and I think there's a lot of design space for multiple weapons (not just a shotgun). Like maybe you have a machine gun that rolls for how many shots are in a burst instead of spread.
I do think the two dice should be balanced rather than completely random, however. Like maybe they always add up to 7, so if you roll a 1 on ammo you get 6 for spread. It felt very fun getting two sixes and not fun getting two ones.