We all know that some have converted S&W (and I think, some Webley WG) revolvers to .45 Colt. It usually works, if the .45 loads are kept at or near std. factory pressures.

But what were/are the pressure differences between the two cartridges?

S&W did some of these conversions, so they must have felt that .45 Colt ammo was safe in converted .455's.

Is the Federal .45 ammo with 225 grain lead SWC-HP bullet a normal pressure load?

I have a converted .455 Triple Lock and I just load the 45LC to the starting loads published.I have never had a problem and the gun shoots great.

The 455 was speced to a max of 13,000 psi and the 45 Colt was 14,000 psi. I would think that it would depend on how well done the conversion was and how well the colt rounds fit the chamber.
Chip King

"The 455 was speced to a max of 13,000 psi"

Can you provide the source of that information? Is that a SAAMI maximum average chamber pressure or from someplace else, such as CIP. How is it measured?

Here is a good link with a lot of info on converted 455's:CAUTION RE .455 REVOLVERS ALTERED TO SHOOT .45 ACP and .45 Auto Rim in British Military Revolvers and Other Handguns Forum

EDIT:scroll down to see info on 45LC conversions.

DWalt there is no SAAMI spec for the 455 that I can find. If you go to this link .455 Webley - Wikipedia, the free encyclopedia you will see the data I quoted on the right side.
Saw the warnings about the 45 ACP and auto rim but both are speced at much higher max pressure levels than the 45 Colt which was what the original poster asked about.
Chip King

My concern is that there are chamber pressures, and then there are other chamber pressures. As the .455 is an obsolete round, I suspect that any maximum average pressure specifications that are published may well be stated as CUP, and not PSI. CUP and PSI are in no way comparable, although it is widely assumed that the peak pressure in PSI is about 120% of CUP. Even though using the CUP method is still acceptable under SAAMI specifications, I doubt that any ammunition manufacturer in the world still measures pressures using the CUP method, as vastly superior methods are available.

Another thing to remember is that maximum chamber pressure specifications are just that. Ammunition manufacturers normally load to peak pressure levels which are somewhat below maximums established by SAAMI or CIP.

Therefore, I don't know if the original question can be answered unless there can be an apples-to-apples comparison of actual .455 v. .45 Colt peak chamber pressures as measured identically. I doubt that information even exists.
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One thing I can say with certainty. Loaded with the same propellant and the same bullet weight to the same muzzle velocity, the .45 Colt will always have a lower average peak chamber pressure vs. the .455, due to the considerable difference in case capacity between the two. Also, it will take more of that propellant in the.45 Colt case to reach the same muzzle velocity.

I've seen 455 caliber revolvers that have gone through the British Proof Houses post 1954 have '6 Tons /per Sq In' marked on them for MkII 455 cartridge.

One British Long Ton is 2240psi (I believe),,so X 6 = 13,440 psi.
That would be the 'Service Pressure' of the cartridge.

How and who converted a Long Ton to 2240psi I don't know and if there are some other conversion factors involved in coming up with the 'psi' rating as we usually interpret it when we think of a SAAMI number,,I don't know.

Just my .02,,
and why I never shoot 45acp commercial rounds (rated at 21,000psi service pressure) thru a converted Webley.

I did some searching and found that at least on the CIP system (European version of SAAMI), the maximum statistical individual chamber pressure given for the .455 MkII (265 grain bullet) is 1035 Bars = 6.7 Long Tons/sq. in. = 15011 psi. CIP Maximum Average Pressure (MAP) of 900 Bars translates into 13,050 psi. The SAAMI Maximum Average Pressure (MAP) for the .45 Colt is 14,000 psi. SAAMI provides no information on the .455.

Therefore, if the revolver is safe for the .455 MkII, it should also be safe for the .45 Colt, especially considering that factory loads will be somewhat below those limits.

At normal pressures for the cartridges in question, the old units would be Lead Units of Pressure (LUP) rather than Copper Units of Pressure (CUP). For the slugs to be able to give an accurate comparison of pressure, the pressure has to exceed the plastic point of the metal. Normal pistol loads won't provide enough pressure to compress the copper slugs, so lead slugs were normally used with pistol and shotgun cartridges, and copper slugs were used for rifles.

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Tom
1560

LUP is (was) used only for very low chamber pressures, such as shotgun shells. Makes very little difference now as both CUP and LUP are essentially obsolete. Those could never measure peak chamber pressures, but sort of an average pressure due to a long response time. Piezoelectric gauges have been in use for over 40 years and are vastly superior as they provide real time instantaneous pressure measurement.

When the guns were made, the .455 was still loaded here, certainly as the MK I version, also called .455 Colt. That data could be compared to that then used for .45 Colt, if we had those figures.

CIL/Dominion in Canada also loaded both rounds, but I don't know if they used US or British pressure measurements.

The 1939 Remington ammunition catalog is the last in which the .455 MkII cartridge (265 grain lead bullet) is listed. It shows a MV of 600 ft/sec.

The Federal 225 grain lead SWC-HP is a standard pressure load, as is the Winchester Silvertip.

It would be unlikely that you would find many old factory records from ammo companies. Looking at a 1955 Lyman Reloading Handbook, the loads of the day seem hotter than current standard reloading information of today.

Reloading the 455 with 225 grain lead bullets show a low of 700fps and a max of 800fps. 45 Colt reloading information shows 180 grain Lead bullets from 900fps to 1200fps, while a 237 grain Lead bullet ranges from 780fps to 1050fps. Unfortunately, these old manuals did not show pressures for any loads.

Bottom line is that I think most 45 Colt loads result in higher pressures than 455.

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Gary
SWCA 2515

Depends on the parameters chosen. As I said earlier, if the .455 and the .45 Colt are loaded with the same bullet to the same MV, the MAP of the .45 Colt will be less than the MAP of the .455. For all intents and purposes, at least considering standard factory loads, there will be insignificant differences between their MAPs.