Ok, on confirmation that certain Battle Magic bullet mutation is possible, I've come up with more ideas for the guns. Same rule though: only one spell affecting the bullet at a time, since more mutation is too much for the bullet to handle.
First off, honourable mention to The Amber Spear. Fantastic spell and would be amazing if we could put it into bullet form. Even if we could, though, it'd be only as amazing as if we got an item loaded up with it; its effects completely obsolete the bullet.
My first idea is a gun with Okkam's Mindrazor. This is a spell that by default affects an entire unit's weapons which is a plus. However, those weapons are, by the rules, exclusively on melee weapons. Might not work if we put it on a gun, but who knows? But if it does work, holy moly is this great. It makes it so that the weapon strikes with the bearer's Leadership instead of their Strength. At Mathilde's Leadership, it'd be striking at effective Strength 8 (Star Dragons hit at Strength 7 for comparison), and since pistols have Armour Piercing that's enough to defeat all armour - even the Master Rune of Gromril. Furthermore, it needing to hook up to the wielder's mind doesn't matter because Mathilde is a shadow wizard. Finally, it may come prepackaged with Smoke and Mirrors, letting the wielder teleport with every shot. (Note: I find this unlikely enough that I'm not counting it in my final evaluation.) Two downsides compared to other spells. First: this isn't a magic missile so no supernatural accuracy and no 24" of range. Second: cost. This thing has a casting value of 18, meaning it'll cost 18 College Favour to get it.
Final evaluation: Get it. Get it for at least one of the revolvers and maybe one or both the other guns too. This is the one thing expensive enough and good enough to warrant spending 5 CF on an Oh Dear tower and learning the spell.
Shem's Burning Gaze is...an option. Not a bad option, just a really frustrating one because its unboosted casting value is 5. It'd cost the same CF to get that as a Battle Magic spell with double that casting value. It makes the monkey part of my brain think I'm wasting 5 CF. And what makes it worse is that its boosted casting value is 15, which is expensive enough you have to start asking if you really want to enchant a gun with this. Anyway. Unboosted is range 24" (double the range of normal pistols, dwarf or otherwise), supernatural accuracy, flaming attacks, and double damage vs undead and daemons. Boosted doubles the range and adds 2 to the bullet's Strength (Moon Dragon Strength).
Final evaluation: Supernatural accuracy, doubled range, and double damage vs undead are good on their own. Worth 10 CF honestly. Might be worth spending an extra 5 for something with the bite of a dragon and a whole bunch more range; if you want to kill undead, it's better than Okkam's Mindrazor while still being cheaper. The boosted version is wasted on the marksdwarf pistol.
Banishment is another Lore of Light spell that's got potential. It starts off good by how its casting value is a perfect 10 - no "waste", no spending beyond the minimum. On its own, the unboosted spell gives 24", supernatural accuracy, double/third more damage (see later) vs undead+daemons, and re-roll successful ward saves. Boosted just doubles the range to 48" and costs +3 CF. The mystery here is the hits. The vanilla spell does 2D6 hits instead of the usual 1D6 hits, so maybe the bullet counts as doing 2 hits or maybe just 1. Lore Attribute increases hits against undead and daemons by 1D6, for a total of 3D6 for this spell, so the bullet may do double damage or a third more damage depending on how it goes.
Final evaluation: Don't both with Shem's Burning Gaze if you're going with the unboosted version of that spell. Flaming attacks is not as good as re-rolling ward saves. EXTRA not worth it if a Banishment bullet counts as two hits. Things change when comparing the boosted versions. Strength 6 is better in most circumstances than re-rolling ward saves and worth the extra 2 favours. Gets iffy if the 2D6 hits apply to the bullets.
I've gotta be honest, at first I completely ignored Searing Doom as something worthwhile. I mean, can't even wound zombies? Get out of here. Then I remembered that this isn't the spell on its own - it's attached to a bullet. Just like that I started rethinking things. 24", supernatural accuracy, ignores armour, flaming attacks, and wounds based on armour back up by a floor of Strength 4, all with a pleasant "just-so" casting value (and thus price) of 10. Boosted version doesn't matter.
Final evaluation: Pretty damn good for just 10 CF. The fact that the pistol gives it a Strength 4 floor makes this the single best example of a spell benefiting being attached to a pistol rather than being cast on its own.
Urannon's Thunderbolt is another spell with a lovely casting value of 10. It's got magic missile's standard 24" range and supernatural accuracy, and it's got the Lore of Heavens' attribute so it does an extra Strength 4 hit's worth of damage to flying targets. What I consider its main effect is that it's Strength 6, which is pretty hefty for a casting value 10 magic missile. The boosted version just ups its range to 48" for an extra 3 CF.
Final evaluation: The lore attribute is near worthless to us, meaning its only real selling point is Strength 6 at a CF cost of 10. It's not a bad selling point, though. I think it's better than unboosted Shem's Burning Gaze since that extra Strength can be useful against more than undead and daemons. Compared to Searing Doom, Strength 6 with armour-piercing is enough to beat most forms of armour while being able to wound shielded Vampire Lords just as easily. Banishment's trickier to beat. Double damage alone is enough to give Strength 6 pause, but perhaps not beat it given that Strength beats both Toughness and armour, but the ward save re-rolling and anti-undead bonus damage serve to reinforce Banishment. Also, boosted Shem's Burning Gaze beats both boosted Urannon's Thunderbolt.
Regrowth. Resurrect people by shooting them.
Final evaluation: The Life Side is a pathway to many abilities some consider to be unnatural.