LeMat - good enough for Johnny Ringo, good enough for players/NPCs.
Re sourcebook - nope, but as stats go it is easy to describe - combine stub-revolver, just change it to 9 shots (as described by Abnett) + single shot shotgun with half range.
In our game, Kolt Thunderer Revolver
The Thunderer is rumored to based on very old, maybe even Terra design. It matches a stub revolver to a undermounted single shot shotgun. While it a robust design, firing the undermounted shotgun imposes a –10 penalty on Ballistic Skill Tests if not used with two hands. Using the butt you can use it a weapon in melee if you have too (d5-1+SB).
Class: Pistol (Solid Projectile)
Range: 30m 15m
ROF: S/--/-- S/--/--
Damage: 1d10+2 I 1d10+4 I
Penetration: -- --
Clip: 9 1
Rld: 2 Full Full
Weight: 2.25 kg
Availability: Rare (Very Rare outside of the Golgenna Reach)