There are Runesmiths that'll rune guns, but the Rune of the Unknown is melee-weapon only.
It depends on how broken you want your bullet to be.
In the end, a gun only function is to throw the bullet at
'sufficient velocity' (ba dum dash).
[Rune of the Unknown: when held by its owner, at a thought it can disappear, and will reappear again when desired. If not currently disappeared, weapon can be summoned to the owner's hand from a distance of up to several hundred yards. Kragg thought he'd never find a use for this rune because no Dwarf would trust their weapon hidden away somewhere they couldn't see.]
if not currently dissapeared, it can be resummoned from several hundred yards. A skinflint assasin (us), can reclaim the bullet if the kill is made from several hundred yards. In all our quest, this is 90% of our engagement range.
When you make a single bullet with this rune, or all three rune in our sword for sufficinent killiness..
[Master Rune of Kragg the Grim: each blow delivers as much force as required to cut or shatter whatever it is struck against, up to the approximate impact force of a cannonball.]
[Rune of Superior Skill: any foe struck by this sword has all magical weapons, armour, items, and positive magical effects cease working for several minutes. Though an original rune, the effect is very similar to Thorek Ironbrow's prototype rune. Knowing Kragg, this is almost certainly deliberate.]
[Rune of the Unknown: when held by its owner, at a thought it can disappear, and will reappear again when desired. If not currently disappeared, weapon can be summoned to the owner's hand from a distance of up to several hundred yards. Kragg thought he'd never find a use for this rune because no Dwarf would trust their weapon hidden away somewhere they couldn't see.]
All three means:
1. It can pierce with the force of a cannonball
2. it can eliminate all kind of magical shield/armor/aura, so you can kill everyone.
3. you can reclaim the bullet, provided you're in range.
If you need size for all three rune, then perhaps the bullet need to be 50 cal or 20mm cal range. not imposible for a ulgu enhance witch to use.
It's practically a game breaker.