I just skipped back through the postings and see that the R-M in question has a cylinder actually chambered for .44 Colt cartridges. Nice! My R-M's are marked .44 Colt, but the cylinders chamber .44 S&W Special cases. That being said, back to the .44 S&W Russian cartridge cases. I use them exclusively to replicate, as close as possible, the .44 Henry Flat central fire cartridges of the day. I also shoot the same loads in my 1860 Henry carbine with a modified carrier block. There's no way to work around the .429 bores, so the .44 S&W Russian cases get me as close as possible to the Henry Flat cartridge.
Johnson Barr--The reason your R-M cylinder/and or frame is marked 44 Colt, but will chamber 44 Special is because at one time Uberti made them that way. Back in 2007 when I was contemplating buying a Open Top and Win Model 66 in 44 Special, one of Cimarron's main guys (can't recall his name, he's still there I believe) advised me that the Open Tops and Richards II and R-M revolvers were marked 44 Colt, but would chamber 44 Specials. This was due to some of the "period correct" crowd wanting to shoot 44 Specials, but didn't want 44 Spec stamped on the revolver. How dare they!!! He advised that the revolvers' of course could handle of the the shorter length brass 44 Colts with ease. Bought both the OT and 66, OT was marked 44 Colt, but chambered the 44 Spec with ease.
Around a year later in late 2008, I bought a Cimarron Uberti Richards II and it was stamped 44 Special, as is a Richards-Mason I bought 3-4 years ago. I don't know when Uberti started and stopped this practice, I know it was stopped sometime between summer 2007 and Dec 2008. Initially I loaded and shot only Black powder rounds in 44 Colt Caliber and loaded Specials with smokeless for identity ease. After a while I just loaded and shot only 44 Colts with Black, still pretty much only do, shooting Specials in a Ruger Super Blkhawk and Marlin 1894 Cowboy lever gun chambered in 44 Mag. Those specials are marked on the box Ruger and Marlin only. I occasionally will load some mild 44 Spec with smokeless and shoot them in either my OT, Richards II, or RM (all chambered in 44 Spec.), even loaded some 44 Spec with black, but didn't see any advantage over the black loaded 44 Colts (25.0 grains FF Colt and 27.0 grains FF Special).
I've run both 44 Colts and Specials through the Uberti Win 66 lever gun. Even though its chambered and stamped 44 Spec, I have no problems with the 44 Colts. Have yet had any feeding or ejection problems. Note: I don't try to slam the tube full of rounds through the action like it was a semi-auto. I can lever them through pretty fast with no problem. I did smooth things up sometime after I shot alot of rounds through it, it's a slick shooter, all stock. If I want a mad moment, I'll load up one of my AR's, Mini14 in 5.56mm, M1 Garands 30/06, or one of my Remington 81 rifles in 300 Savage and spray and pray to get those kicks. Now that's centerfire fast shooting.