Ruger has done several limited-production runs of 38-40 single action revolvers, both Blackhawk (adjustable sights) and Vaquero (fixed). These were usually double-cylinder guns with one cylinder in either 40S&W or 10mm and the other in 38-40.
http://gunsamerica.com/search.cgi?l...ge=0&uid=&words=38-40&category=2300&state=All
There's two Vaqs in 40S&W/38-40 combination and one in 40S&W alone.
If you find a good deal on a 40S&W or 10mm gun, the 40S&W cylinder can be inexpensively converted to either 10mm, 10mmMagnum or 38-40; 10mm cylinders can be converted to 10mmMagnum or 38-40. You would only need to ship the cylinder off to any reasonably competent gunsmith who has a 38-40 chamber reamer.
Another option:
Say you have a 40S&W Vaquero and you don't want to give up the ability to use that very common cartridge, but you also want 38-40. Take the gun to a gun show and look around for used Vaquero/Blackhawk "New Model" cylinders in some caliber smaller than 40/10/38-40. The 357Mag, 9mm or even 30carbine would do. Check the endplay, cylinder gap and timing fit on your gun until you find one that works - use the "revolver checkout thread" procedures to check fit, they'll work even though it's the wrong caliber. On average, the odds of a given Ruger New Model cylinder fitting another gun without timing or endplay mods with all parts in good condition is around 50/50 and with these guns so common, one or two gun show trips should net you a good candidate cylinder. Send that cylinder off to the gunsmith for reaming to 38-40 and bingo
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The final option is a full caliber conversion up to 38-40 starting with a common 357 or whatever...and this could be done in just about any revolver that's got enough cylinder "beef", such as a DA S&W N-Frame six-shot 357 or a 357 single action by USFA, Beretta, Uberti, etc. Ream the cylinder bigger and swap barrels...any decent gunsmith can handle that, although prices are significantly higher than just a cylinder ream. Still, an S&W model 28 converted to 38-40 would be way cool and very weird
. It *might* be practical on an S&W L-Frame (six shot, not seven!) platform or the GP100...you'd be pretty low on cylinder wall thickness but then again it's a low-pressure cartridge...consult a competent gunsmith before going that route.
NOTE: in some cases, Ruger has shipped "double cylinder" guns such as 44-40/44Mag or 357/9mm where the barrel specs are a close but imperfect match, leading to one caliber or the other shooting poorly with at least some ammo. This is NOT the case with the 40/10/10Mag/38-40 family - they really do all use the same barrel so there's no accuracy compromise.