My 625MG in .45 Colt measures just over .480" ID in it's chambers. My leftover Hornady .454 Casull ammo measures .477" OD, as does my GA Arms .45 Colt ammo - and my homebrews, made using Lee carbide dies - which I also used to reload my .454's with. The simple truth re the .480" maximum OD of the .45 Colt is based on it's initial propellant - black powder. Being very dirty, some allowance for same was included.
I shot thousands of .45 Colts through my 7.5" SRH with no problems, save the embarassment of being bested, grouping wise, initially by my 4" .45 Colt 625MG, and later by my 5.5" RH. No, standard SAAMI spec-ed .45 Colt loads - especially cowboy-lite - will not normally obturate the case to seal off the chambers (Witness the smoke trail usually on the top of the fired case.). Combine that with the low velocity of the .45 Colts (700-850 fps) - and the 'slow' rifling in the .454's barrel (Designed for 1,600-2,000 fps), and one can uderstand the poor performance of .45 Colts in my 7.5" SRH. While I could get 1.5" 5-shot groups (I allowed a 'flier'...) - with it scoped from sandbags - at 50yd, it wouldn't do better than 4" at 25 yd and 6" at 50 yd, under similar conditions with .45 Colt.
My .45 Colt-ish loads in .454 cases did better - and had lower SD in muzzle velocity, too. I believe this is more a function of the case filling the chamber length to the case mouth step, something that usually doesn't occur in some .45 Colt chambers. I have lost the data taken, but I remember <3" at 25yd and <5" at 50yd, and mv in the 920-950 fps range, using my usual 250gr LRNFP's and 255gr LSWC's. Again, I think the 'slow' rifling didn't help.
One can't over-emphasize that you MUST shoot .454's first, then .45 Colts during a shooting session. Always meticulously clean the chambers before going back to .454's. I knew this - but still destroyed a favorite plinker, my 629MG, last February. After shooting hundreds of .44 Russians and Specials, I wanted to ping a 100yd plate one more time. I loaded - with some effort - my sole remaining ammo - 300gr LSWC over 6.2gr Titegroup in a .44 Magnum case (~880 fps). I had shot many of these heavy plinkers before. I hit the plate with the fourth shot - and destroyed the revolver with the fifth. The bullet couldn't exit it's case due to the lead and carbon buildup at the step - the crimp couldn't release. The resultant pressure spike burst the case, cylinder, and topstrap. A 'double charge' was ruled out, due to the small volume with that long bullet loaded. S&W returned the serviceable part - the barrel - and agreed with me and Hodgdon's re the way in which it was destroyed. It was all my fault. Believe me, crud in your chambers from fired short-cased rounds can be dangerous. Fortunately, I only had to mop up my tears, get another pair of safety glasses (Chipped lens!), and change my britches.
A great way of cleaning excessive carbon/lead from your SRH's chambers is with a wool mop and metal polish (Flitz/Semichrome) powered by a slow drill motor. This will help with softer-cased .454's, too - like the early Hornady's and MagTech's. Fun revolver - I actually miss mine - although my carpal tunnel problems don't!
Stainz