Was just thinking about what feats you'd need to become a supreme crossbowdwarf, and here's what I came up with for a fighter.
In no particular order.
Point Blank Shot, Rapid Shot, Rapid Reload, Precise Shot, Improved Precise Shot, Dodge, Mobility, Shot on the Run, Weapon Focus(Ranged), Weapon Specialization(Ranged), Greater Weapon Focus(Ranged), Greater Weapon Specialization(Ranged), Superior Weapon Focus(Ranged), Improved Critical: Ranged, Power Critical.
This all gives you:
+3(+4) attack +4(+5) damage (under 30 feet) Fast firing, firing while moving, shots damaging all enemies in path, improved rate of successful criticals.
Leaving 3 more feats free. Should also do this after true reincarnating from a ranger to get Past Life : Ranger, for another +2 damage and also from a fighter to get Past Life : Fighter, for +1 attack. If I'm reading the descriptions correctly each of the past life feats stacks up to three times, in which case the crossbowdwarf should come as the seventh incarnation. This of course requires reaching level 20 six times before so it will take quite some time. Past Life : Monk doesn't say if it effects ranged damage but if it does that would be another to add.
Haven't particularly looked at enhancements for it but the fighter Kensai series should probably be part of it, for improved attack, damage and criticals, as well as the fighter capstone enhancement for faster attacks.