The feat Spell Cartridges allows your gun to shoot force bullets instead of regular ammo. These bullets count as magic for bypassing Damage Resistance. But something bothers me: if these are Force Bullets shouldn't they be affected by Energy Resistance (although Force Resistance is extremely rare) instead? And in such case shouldn't they be affected by Spell Resistance too?
According to Michelle A.J., Pathfinder Contributor,
Do the force bullets overwrite the base damage of the gun (so your pistol and rifle both do 1d4/five levels, plus usual mods like deadly aim) or is this damage added on top of the base damage (so you do eg 1d6 + usual mods + 1d4/five levels)? I'm guessing it's probably additional damage, since the feat says they bypass DR/magic but force damage isn't subject to DR in the first place.
I believe that the D4 replaces the base damage of the firearm, it says you fire "force bullets" instead of regular ammunition. So your base damage at 5th level would be 1D4(Force bullet) + 2(Arcane Strike) + relevant bonuses. 10th level, 2D4 + 3 + bonuses. One other thing the feat is unclear about is if these force bullets need to be loaded into the gun or not.
The damage from the force bullet replaces the base damage of the firearm. The clause about bypassing DR is mainly to keep it consistent with Arcane Strike.
And force bullets do not need to be loaded. Using Arcane Strike as a swift action "loads" the gun.
Whether or not you want to take the forum post of Michelle A.J., Pathfinder Contributor as “official” or not, this could answer your question.
Certainly, from a rules-as-written perspective (which would generally not treat a forum post as official, since it doesn’t go through editing and isn’t widely accessible), things “made of force” deal “force damage;” that has to be true for many, many features. So RAW, you are correct, the force bullets ignore DR, and the statement about being magic for DR purposes is meaningless.
On the other hand, it’s possible to imagine something “made of force” that actually deals bludgeoning, slashing, or piercing damage, and not force damage. This would be very strange, and without precedent in the rules—as far as I’m aware anyway—but it would make the line about DR make sense. This isn’t RAW, and isn’t backed up by any developer commentary I can find—not even a random forum post—but if Spell Cartridges were overpowered I could see going with it.
But Spell Cartridges is not overpowered, even as force damage, I think. It’s a strong feat, and Arcane Strike isn’t a terrible feat either, but two feats is still a large cost—and an even bigger “cost” is the mere fact that you’re using Pathfinder firearms in the first place, as they’re pretty terrible. Arcane Strike and Spell Cartridges overcome a lot of those problems, but the cost of a swift action every turn isn’t negligible either. So my suggestion is to go with what (little) information we have, and treat them as dealing force damage. It will be good, but feats should be good.
And in such case shouldn't they be affected by Spell Resistance too?
Spell resistance really is just spell resistance. Only spells and spell-like abilities care about it; all other forms of magic, including the Spell Cartridges, ignore it.