I have been pretty lucky with conversion cylinders. Got my first one back in 2000 from Kirst, a 5 shot cylinder and it dropped right into my 1999 Pietta Remington with no work needed. In 2003 I bought a .36 cal Remington and purchased a 6 shot Howell (R&D) conversion cylinder in .38 and had the barrel relined to .357. Again total straight drop in from the factory with no smithing needed. The Howell's and Taylors cylinders are identical. A few years later a I obtained a Kirst 6 shot conversion cylinder for my Ruger Old Army and it worked flawlessly right out of the box.
In 2005 bought a 5 1/2 barrel Remington like the Captain's and recently bought two cylinders for it, a Kirst .22 cylinder and Kirst's new "Equalizer" in .45 Colt. I had a problem with fitting and timing on both cylinders so I contacted Kirst's gun smith, Bill Kelly, and he told me he was confident that he could fit and time all three cylinders, cap & Ball, .22, and the .45 to work in one gun, the 5 1/2 barrel Remington. So off the gun goes to him.
I recently purchased an 1851 .44 Yank Snubnose and my Kirst Colt .22 conversion cylinder dropped right in, no smithing needed.
Pictured below are two of my Remington's, with an early Kirst.45 Long Colt cylinder on top and below that my .36 Remington with the Howell's .38 (R&D) cylinder.