With some practice, while I'm not saying they have any real practicality, they can be surprisingly accurate.
Shooting from the hip at a B27 silhouette, I got fairly good with my first one, the Chiappa .44-40, out to 5 yards or so.
Shooting that one with sights in a two-handed hold, it'll easily take down my small steel buffalo out to 30 yards, that's as far as I tried that one. Using it in a rifle position against my shoulder I suspect it could run out to 50 yards or so.
The most recent, the shorter .45 Colt octagon barreled Chiappa without sights, was grouping at 2-3 inches off a rest at 25 yards.
With NO SIGHTS, it's more intrinsically accurate than many true pistols I've worked with.
In a two-hand hold, I've also knocked my little steel buffalo down at 35 yards with it.
Not hard at all.
Each scene where McQueen used his Mare's Leg was carefully staged around it.
The gun is not particularly practical, but it CAN be surprisingly accurate, and McQueen COULD have hit most of what he was "shooting" at on the screen.
Denis