I do not recommend shooting standard 45ACP ammunition in these pistols as they were not built to withstand the pressures.
I have one of these and after testing it with a number of handloads, I am going to continue to use the lightest ones.
Whoever did the caliber conversion did it cheap and reamed the chamber mouths oversize.
I have no idea of the diameter of the barrel, but it must also be wide. I found .454 diameter bullets shot OK with a little leading. Smaller bullets gave more leading. I don’t have a mold that casts bullets .457, the larger dimensions are probably more appropriate.
I thought I would use some loads I developed in my 45 ACP revolvers. I had some Elmer Keith loads of a 250 with 6.5 grains Unique. That load was way to hot in a Webley. Actually stretched the top strap recess!
After that happened I realized that this mechanism is a lot weaker than any solid frame I have ever handled.
I also reconsidered using Elmer Keith loads in any of my 1917’s. His loads are entirely too hot for vintage pistols.
I eventually decided that 3.5 grains Bullseye was a good load. It shot to point of aim and was easy on the pistol.
Code:
45 Auto Rim
455 Webley MkVI rechambered 45ACP
manufactured Enfield 1923
255 LSWC (.454) 3.5 grs Bullseye thrown, R-P AR cases, CCI300 primers
Jun-Aug 02
Ave Vel = 541
Std Dev = 10
ES = 42.44
Low = 522
High = 565
N = 24
Shot to point of aim 25.0 yards
255 LSWC (.454) 4.0 grs Bullseye thrown, R-P AR cases, CCI300 primers
20-Jan-02 T = 42 °F
Ave Vel = 640
Std Dev = 21
ES = 79.1
Low = 586
High = 665
N = 12
Shot to point of aim 25.0 yards
255 LSWC (.454) 4.5 grs Bullseye thrown, R-P Auto Rim cases, CCI300 primers
20-Jan-02 T = 42 °F
Ave Vel = 681
Std Dev = 8
ES = 28.4
Low = 667
High = 696
N = 12
Shot to point of aim 25.0 yards better accuracy