Howdy, Pards,
Ruger messed up rather badly with the .44-40's. As was posted, they went with .429" groove diameter barrels. That wasn't so bad, in and of itself, as most of the Italian-made .44-40 guns have .429" barrels.
Where Ruger missed the boat initially was in following the SAAMI standard for BULLET diameters in the .44-40 as the standard for chamber and throat diameters. SAAMI calls for bullet diameters of .427 + .000, -.003 or .424-.427"! At the time Ruger brought out the Vaqueros in .44-40, the ONLY factory ammo was made by Winchester and Remington, and both use .425" dia. JACKETED softpoints! (Back in the 1970's, Winchester's jacketed softpoints were .427".) Dispite being told by a number of noted gun writers, and ersatz captain of 1870's cavalry, that Cowboy Action shooters would be the primary market for these guns, and we'd be using cast bullets, Ruger insisted on making the chambers tight and the throats .425" in diameter.
After much adverse comments in the press and on the phone, Ruger agreed to open the CHAMBERS to accept at least Winchester brass with .430" bullets. But they insisted on leaving the throats at .425"!
Finally, just before discontinuing the .44-40's altogether, they opened the throats up to .431 or so. (Don't know personally!)
Frankly, I found the "2nd issue", larger chambers, .425" throats, guns will drive tacks (1-5/8" groups at 25 yds, eyeballs co-operating and good lighting conditions). But you have to use .430" hard-cast bullets (most commercial bullets will do nicely), and W-W brass. (Starline brass is a good alternative; Remington is too thick!)
Only problem with Winchester .44-40 brass is scarcity and I am starting to lose a few. Can't seem to get over 17-18 reloads anymore! I take the oldest up to Hell-On-Wheels as they are now a "no-brass" match!