When you finish a long rest, you can use woodcarver’s tools to carve special sigils into a wand, staff, or rod and thereby turn it into your arcane firearm.
While this doesn't say that it makes a musket...what's stopping you from turning an engraved rod into a musket after? You say 'a wand with special sigils carved into it,' I say '30 inches of filigreed ash-wood shaped to fit my shoulder and hold a metal tube'. Is not a wand with metal attached to it still a wand, my friends? I, for one, welcome our finely decorated arquebuses and jezzail muskets of arcane destruction!