The barrel on the 44 conversion appears to be an original Ruger Speed-Six barrel, bored oversize.
I just measured three different Speed-Sixes....9mm, 357, 44special. Interesting findings.
The cylinders are all slightly tapered toward their distal end. I measured proximally at the widest point, over the bolt notches. Each one measured identically @ 1.497".
The cylinder lengths varied by less than .01", averaging 1.603".
The cylinder window heights for the three varied only slightly:
9mm/1.533", 357/1.532", 44sp/1.546"
The cylinder window length, from recoil plate to barrel breech all measured @ 1.674".
It does appear that there is more clearance between the closed cylinder and the bottom side of the top strap on the 44 than on the other two...as if a tiny bit of metal has been removed. But, what with the cylinder diameters all being the same, I don't know why that would have been necessary for the conversion.