Well... to make gunpowder, you need saltpeter, charcoal, and sulfur. Sulfur can be found naturally, charcoal can be made easily, but potassium nitrate (saltpeter) was historically obtained through a process involving manure, wood ash, and long periods of time. Another method involves processing urine over a shorter period of time. I don't know how engineering could fit into this.
I suppose actually producing guns would be engineer's work, but gunpowder is another story. Have we really been using engineering as the skill to make gunpowder?
For characters who can't make their own gunpowder, they require the assistance of an engineer, engineer basically means scientist. It's been that way since You are King I, since that's when gunpowder was invented (though it was really never used, and guns/cannons did not exist) I should know I'm the guy who invented it in this world, and now I'm his grandson. So yeah it is technically chemistry, but it's governed by the engineer skill. I'd ask a gm about your chemist skill thing since it's kinda pointless unless for making poisons or poultices I guess.
Plus it takes 2 entire seasons to make 1 unit of guns, and another season to make 1 unit of gunpowder, so it's not like you'll be much help in the battle in that way.
So it takes 3 seasons to make 1 batch of rifle and then it'll probably cost even more to train and hire the men to use them, so guns are expensive, I hope their worth it. Oh also you have to get your hands on some gun blueprints, which you can either buy or spend some time experimenting and making your own, which all of this costs you money.