I have some experience loading for the .455 and thought that I would share what I have learned..
The .455 Mk1 introduced in 1891 used a 0.88" long case and 265-grain hollow-based bullet of 1 to 12 tin-lead, lubricated with beeswax and propelled by a compressed charge of 18 grains of black powder.
The Boer War Model Mark IV revolver was the first proved for use with Cordite. While some pre-1900 revolvers were refitted with heat treated cylinders during Factory Through Repair for WW1 service unless your older MkI, MkII or MkIII revolver was reproved and marked for 4.5 Long Tons (10,080.psi) smokeless loads should not be used in it at all!
Many Webley revolvers exported to the US after WW2 had the rear of cylinders faced off 0.06" to use .45 ACP with moon clips. While factory lead .45 Auto Rim maintains a machine loading limit of 14,000 cup, in deference to the shaved Webleys, WW2 USGI ammunition exceeded .455 proof pressure and has been known to blow cylinders or loosen guns up, shearing the barrel catch screw.
The commercial .455 Eley and .455 Colt cartridges used the same 0.88" length case as the Mk1, but were loaded with either Cordite or flake smokeless powder, producing 700 +\- 25 fps with 265-grain lead HB bullet from a 5-inch solid industrial test barrel. Canadian CIL commercial ammunition was loaded to these specs until the mid-1970s. Revolver service velocity in the longer CIL case is approximated with 4 grains of Bullseye or 5 grains of Unique with 255-265-grain bullets seated to a minimum overall cartridge length of 1.25". While some old data sources list heavier .455 loads for the S&W Hand Ejector and Colt New Service, these charges should not be exceeded in the British military top-breaks.
Much older .455 load data from Waters. Lyman and others is intended for solid frame, swing-out cylinder revolvers, being assembled in the longer CIL cases. These generate excessive pressure for use in the British military top-breaks when the shorter 0.76" Mk2 cases by Fiocchi, Hornady or Starline are substituted.
Modern Fiocchi commercial Mk2 ammunition is loaded to CIP standards to obtain 200 m/s (656 fps) from a solid 13 cm (5.13") industrial test barrel at pressure not to exceed 900 bar (13,053 psi). Safe charges approximating service velocity for the British top-breaks can be assembled with Accurate 45-264H and 3.5 grains of Bullseye, 4.2 grains of 231 or WST or 4.5 grains of Unique. A small pistol magnum primer used in Fiocchi cases is a safe substitution which gives the same velocity as standard large pistol primers assembled in the large primer pocket Hornady or Starline cases.
.455 Mk2 Velocity Data - Hornady .455 Mk2 brass, Winchester Large Pistol Primer
Firings in 1914 Webley MkVI with 6-inch barrel:
Kynoch 1942 MkViz 265-grain FMJ. 580 fps, 32 Sd
Lyman #452374 225-grain LRN 5.0 Unique. 648 fps, 24 Sd
Accurate 45-264H 3.5 grains Bullseye. 622 fps, 6 Sd
MiHec 265-grain Mk2 HB 4.5 Unique. 538 fps, 24 Sd
The full-diameter, parallel forepart of the Accurate bullet is intended to fit the cylinder throats and provide positive guidance when rounds are assembled into the shorter Mk2 cases, but fired in the longer .455 Eley/Colt/Mk1 chambers. The large meplat gives nearly full caliber crush giving deep penetration without tumbling and good field performance. Most Webley top-breaks have snug cylinder throats of .450-.452 whereas S&W Hand Ejector and the Colt New Service pin out .455-.456 and prefer larger bullets.
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Originally Posted by NuJudge
