I have a hunch you’re being led astray by the lying character screen. The damage dealt is not necessarily what is displayed here. Having fire mastery would effectively double dip and incorrectly present too high of damage.
From the Amazon Basin:
+% FSD also applies to melee attacks at time of attack, so it will be applied a second time to melee fire attack damage added by Enchant, Holy Fire and items. Character screen displays +% FSD being applied to ranged fire attack damage as well, but this is not actually the case.
Another way to think of it, is that you are not “casting” any fire skill with the demon machine. You are attacking with a ranged weapon. The giant damage boost comes from the buffed enchant. There’s an incredibly small boost of added FSD to the exploding arrow itself, but it’s negligible (lvl 6 exploding arrow itself is ~29 dmg).
If you have time, you can do an experiment on Diablo. See how many bolts it takes with and without the fire mastery and let us know.
Wow so if char screen is lying then this means it is possible to forfeit both my enchant and fire mastery skill points , while also freeing up my invy. I did try to make a nova sorc with a demon machine but just couldn’t get the nova powerful enough as i was a bit short on some of the synergies. if i don’t have to put into fire mastery i might be able to
achieve a hybrid nova / demon machine sorc. i’d carry around lite lifers as well as a fire and lite sunder