Oyeboteb, I have a New Service much like yours, still in its original .455 Colt chambering. (".455 Colt" is what Colt called the cartridge; I doubt the Brits ever used that designation, but that's what Colt put on the barrel).

IIRC, I shortened some .45 Colt cases in an attempt to see if they would function without thinning the rim. I found that I could put one of these cases in a chamber and position it so it was at the lowest point (farthest away from the barrel) and I could close the cylinder just fine. The problem was, the cylinder would bind when the action was cycled. So the .45 Colt rims are just too thick, at least in my Colt.

I found that .45 ACP cartridges had excessive headspace, and that if I put them in moon clips it was impossible to close the cylinder.

My brother was kind enough to thin the rims of some .45 Colt cartridges and then shorten the cases to the correct "Mark I" length (0.890") and I use these modified cartridges for loading .455 Colt cartridges for this old war horse. It used to be possible to buy similar modified .45 Colt cases from Buffalo Arms, but I haven't been to their website in years so I can't say they are still available. Last time I went to their website, it was really hard to find anything.

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