Sounds like Gene Starwind's gun from Outlaw Star.
I don't think it would be an artifact, just a magic item.
It should have several limitations:
1. The "ammo" used should only be for offensive type spells directed at an enemy or an area - no buffs or utility spells. I would probably just stick with scrolls - expensive but closer to the inspiring source material (I think).
2. All spells should require attack rolls, even area effects spells.
3. For consistency, I would use the firearm misfire rules in UC and treat those as scroll errors.
3. I would increase loading time and add a -4 attack penalty if nonproficient.
4. Applying firearm/combat feats is probably fine, as long as the loading times prevent full attacks. Being able to get offensive spells off as iterative attacks could be pretty devastating.
5. I think attack should be standard action, but reload should be a full round action, subject to being reduced by rapid reload feats.
I love Outlaw Star, so I think the idea is a good one to borrow.