is there a way to make chests sometimes spawn with guns/ammo?

Use custom loot table which generates TACZ gun items that generated with correct NBT?
You can use https://misode.github.io/ for editing a loot table online and then edit it by your own way.
To grab a valid gun NBT data, you could get a gun in your hand in game and use /data get entity @s SelectedItem to see its NBT.