The 200 grain .380 Revolver Mk1 load was not used in combat during WWII precisely because it was in violation of the Hague Convention, and was therefore illegal. However, it was used some for training, and Mk1 ammunition in inventory was used up that way. The only official combat .380 load used in British Commonwealth Victory and Enfield .38 revolvers during WWII by the British was the FMJ 178 grain Mk2 load (but I cannot say there was no surreptitious use of the Mk1 load in combat from time to time).
Presumably the reason for the original adoption of the 200 grain lead bullet load in the post-WWI era was that it was assumed to be capable of producing equivalent "knockdown" performance to the .455 revolver cartridge used by the British during WWI, but in a smaller and lighter-weight revolver. I have never read how the British reached this conclusion, nor why they chose a weak obsolete cartridge (the .38 S&W was one of the first black powder cartridges in the 1870s), instead of a much better caliber, such as the .38 Special which was well-established by WWII, nor why they chose to adopt an obsolete top-break revolver design when far stronger solid frame revolvers were also available. My guess is that they were somehow wedded to the break top revolver design because they knew how to make them, and by so doing, they were then limited to selecting a weak cartridge, as the break top design is in itself weak.
Post WW1/1922, England began searching for a replacement for the heavy Webley MkV1 .455 revolver. The War Office made that decision.
Webley & Scott which had always had close ties with the British Gov;t was commisioned by the WD to undertake development of the new revolver (how delightful,,thank you W/D!)
They offered up a modified MkIII .38 first,,then the new (at the time) MkIV 38.
Both were liked by the Small Arms Committee of the WD ,,probably the modified MkIII a bit better as it is a little lighter and smaller OA.
The caliber 38S&W was a concern of course with it's effectiveness potential.
With commercial ammunition the pistols were very easy to shoot assumed easy to teach recruits to shoot..
The short cylinder length of the Webley kept chambering in 38sp out of the picture. So the step up to the max bullet weight possible was the choice. The 200gr lead.
At 600fps,,adding to bullet weight was all they could do to increase effectiveness over commercial loadings,,the pip squeeks as I labeled them.
It's approval,,the 200gr load,,as a suitable replacement for the .455 was nothing more than a Gov't W/D & Small Arms Committee report and sign off stating that it was so.
I guess we really don't rely on much more than that for most performance tests.
It wasn't till almost 10yrs later that the round was officially adopted 1931 or 32. Arguements over tiny changes in bullet shape, training rounds, blank ammo, everthing imaginable had delayed the project.
Then in '37 it was changed from the 200 gr lead and given an alloy FMJ to meet the International demands of play nice during war rules.
The resulting 178gr weight is nothing more than the result of what weight they ended up with after stripping the bullet of enough heavy lead to apply a lighter weight FMJ covering.
It still was the heaviest, largest bullet of the design style they agreed upon.
Now a FMJ leadcore make-up just a few grains lighter than the solid lead example, and it made everyone that got shot at with it happy because it wasn't solid lead.
S&W H/E's were never in the picture. Short cylindered Webleys were the original gun to fit the cartridge.
Gotta go with the home grown product,,in this case it was the Webley topbreak.
Webley got it stuck to them by the Brit Gov't when the latter took Webleys production examples to ROF Enfield and designed their own revolver.
The Enfield No2MkI was the result and what was adopted. (A Capt. Boys was involved in that development at Enfield as Asst Supt of Design at the time.)
Webley later sued, but received little if any in return. Bad blood between the W/D and Webley after years of very close business relations and contracts. Webley did produce MkIV 38's later in WW2 as the gun starved Nation needed everything they could get.
But the Gov't designed and made Enfield 38 No2MkI beat it out to the line.
