Explosive slugs are pretty useless for what they are worth. Swap out the incendium crystal for a brick of UEX to make the recipe a little more viable.
An additional recipe for the "Don't push it" skill that makes use of incendium could be an incendiary round capable of burning underwater, having a chance to ignite targets from within (deals burn damage over time for 3-5 seconds). It requires Incendium, plastic and phosphorus for 6 shells. Significantly lesser range than other shell types but well worth the expense when dealing with ballast flora or fractals (deals bonus damage/higher immolation chance)
"Arms race" grants depleted fuel ammo for small arms. It would only be natural to include a DF shotgun slug and round out the core 3 ammo types in barotrauma. A singular projectile that can easily drop a mudraptor or mantis.
"Weapon artisan" currently improves your weapon quality by a factor of one. Kinda shit for a tier 3 skill, especially when the only weapons an engineer can fabricate are harpoon guns and the big ol fissile gun. I propose that an engineer with this skill is given the ability to craft weapons that have been unlocked by security or the captain, however it requires the presence of said crew members in the server (they must also be alive and within the sub) for the recipe to work.