Ready for a long winded response????

I don’t even know just where to start when considering .455 conversions….
First, realize that nearly all .455 chambers are designed to accept either the .455 MkII cartridge or the .455 MkI cartridge (aka .455 Colt in Canada). The main dimensional difference in these rounds is case length with the MkI being longer then the MkII. The 45 ACP and 45 AR are both shorter and have a smaller head diameter than the .455 so there is no need to deepen the chamber. A .45 ACP will drop into an unmodified .455 chamber and fall .100 below the chamber mouth. (measured in a .455 TL) The difference in head diameters between the two are around .004 which can cause a little swelling of the case, but shouldn’t be a deal breaker unless loading hot rounds. (which no one should be doing in a conversion in the first place.) The .45 Colt has a slightly larger head diameter so this problem shouldn’t quite as pronounced. At the same time the .45 Colt case is longer and the chambers will need to be deepened in order to accept it.
Now we get to headspace… I can’t find what headspace is recommended for the rounds under discussion, so I am going to consider that normal headspace ranges from about .004 to .013. Anything less than .004 can cause hang-ups in cylinder rotation and anything over .013 can cause case stretching, misfires, and a whole symphony of undesirable results. (Especially with an already too small head diameter.) So, in order to accommodate the thicker rim of the .45ACP/45AR , (the AR being designed to allow its use without clips), so rim thickness can be considered the same for both cartridges. Remember the 45ACP is designed to headspace on the case mouth, not the rim. (1917 S&W had a headspace shoulder built in so one could use 45ACP without clips, but extraction had to be done with a stick. 45AR works just fine.) The .455 chamber does not have this shoulder so all headspacing is dependent on the rim of the case. This means that something will have to give to get either the 45ACP/AR or the 45 Colt to fit. The given remedies are shorten the cylinder, shave the standing breach (recoil plate), rebate the charge holes or a combination of the above. The various rim thicknesses are: .455 = .035, 45 Colt = .055 and the 45 ACP (w/clips)/45AR = .085. To allow use of ACP or AR will require that app. .050” of metal be removed somewhere. However this will also add .050 to the headspace requirements for the .455 which is way over the recommended max. A suggested way around this is to rebate the charge holes to allow the smaller rim diameter of the 45AR (.516) to drop in far enough to headspace and still allow the .455 rim (.535) enough cylinder face to headspace. (This will also allow the 45 Colt rim (.510) to fit if the charge hole has been deepened.) The only problem with this is that it precludes using clips on 45ACP as they will try to headspace on the same surface as the .455, which was the original problem. (Remember, no shoulder for the ACP to headspace on.) If you are willing to give up 45ACP, this is a pretty elegant solution as it doesn’t change any external dimension of the gun, the cylinder stop is still within specs, it retains the cylinder serial number, etc. The only problem is that it will add around .030” to the headspace of the .45 Colt. (The difference in rim thickness between it and the Auto Rim). This is outside the recommended range, but as long as the loads are kept in the lower range, shouldn’t cause many problems. So, if you want to keep the ability to use 45ACP in clips, it’s pretty much shave the cylinder or breech face with all of the inherent problems included, the main one of pretty much writing off safely using .455. (I suppose one could try making up some kind of a .050 “moon shim” that could be placed on the back of the cylinder and have the .455 rounds placed through it.)
Now, I have seen a few .455’s that will close on a 45 Colt rim, but they have to be loose to the point of being way out of spec as the Colt rim is .020 thicker than the .455

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Dean
SWCA #680 SWHF #446