A reusable bullet sounds like a hilarious and awesome way to work around some of the limitations on particular runes. The trick is that you'd have to make it out of gromril and you'd have to spend two-ish runes on making it reusable - one for retrieval (something like the Rune of the Unknown or Flakkson's Rune of Seeking) and one for extra durability (in case you shoot a major demon or a gromril plate or something). Then you can put one more good solid rune on it, maybe a variant on Kragg's Master Rune or an antimagic effect or something. The best part is, since ammunition is normally disposable, a master-quality reusable bullet would be a strict improvement over normal ammo in the eyes of the dwarves!
Economic wise its cheapert then the smallest dagger too.
Irrc gromil is like 10 to 50 times worth its weight in gold. Add to the fact that its practically used as armor, then a bullet weighted only 50 gram will be a more worthwhile investment then a dagger weighting 100 or 200 gram.
The only problem will be the gun barrel. Unless made out of gromril as well, it will wore it out after a few shot.
But this is a perfect setup for a one shot marksdwaft gun that we have (and never use), and anything we shot using this gun means a vip target worth more then dwarven steel barrel.
So far the runes are engraved on the gun itself, thats why theres melee rune and firearms rune and cross bow rune.
I dont think anybody ever think of using the rune on the bullet/arrow itself.
The only limitations are
1. This is a potential gamebreaker. As fun as it is, it would take away the fun or any combat as this is very much like +50 to all combat roll with that gun.
2. Is it even possible in the first place and would somebody be willing to experiment to make it.
Runelord are the very extreme end of traditional.
Kazrik is not master yet, and i doubt those 2 runemaster would make it without a loot of grum ling.
3. You still have to reload the gun with gunpowder, even if you can recall the bullet. If we manage to crack the secret of magnetism, only then it can be a true railgun.
Its a nice what if experiment, but i doubt it will be implemented.