I'm trying out 1 pyrite spellbomb in place of the 4th firebolt as a tutorable creature kill. I searched the thread and no one has tried it before without bomberman combo as well. Someone did mention running a seal of fire, which I like, but this does the same thing (albeit one mana more expensive) and can be welded back and forth for value and additional utility. I am not sure if it is better than the 4th firebolt. Time will tell.

If you are playing mono-red I would not play any pyrite spellbombs. Max out on firebolts.

If you are playing white splash and are running 2 or less welders, running engineered explosives like in Seth's list will give you more flexibility and kill all the same creatures you care about plus other permanents as an upside. For that matter, 2 or less welder lists should probably swap ratchet bomb for Explosives too and keep the full firebolt set. The reason for this is that you are less likely to naturally draw a welder and untap with one alive. You are more likely to gain value from the slot by flashbacking firebolt than you are having a pyrite be an artifact.

Just my thoughts for now.