They were developed to use the old .38 Smith & Wesson cartridge, but with a special load developed from the Webley "Manstopper" soft-lead cup-point bullet.
Original military loads used a rounded-nose nearly-cylindrical lead bullet of 200 grains weight at a bit over 600 ft/sec. This was later changed to a 178-grain round-nosed FMJ type and called the ".380 Mark II Revolver" round.
The same cartridge was also used in all those Webley Mark IV WW2 revolvers and in all the .38 S&W revolvers we bought from Smith & Wesson. Because the normal American loading for the .38S&W cartridge was a 146-grain bullet, the pitch of the rifling was special to these guns. Smith & Wesson referred to the guns made on contract for us as the Model 10-200 in recognition of the different rifling pitch of the barrels.
The guns - all of them - are fun to shoot and they can often be very accurate.
The biggest problem is that they all shoot LOW owing to insufficient barrel flip with the lighter bullets, which are the only ones available. Nobody, it seems, wants to make a 200-grain .38 bullet, despite the very large numbers of these excellent guns which have been dumped on the market. To add insult to injury, nobody even wants to make a 200-grain mould for a pistol; the only 200-grain .358 moulds available clearly are designed for rifle use. I have been suggesting this and b*tching about it for over 30 years and, so far, have yet even to receive the courtesy of a letter telling me to go f--- myself. They just are NOT interested.
The problem is that it was never a US military cartridge, else we would be inundated with bullets and moulds and hundreds of magazine articles proclaiming it the best thing since sliced bread. American manufacturers have never twigged to the little fact that the British Commonwealth ALSO was in that war. After all, the Commonwealth was ONLY a quarter of the world: nothing important at all.
The fact is that the little old .38S&W loaded with a 180-to-200-grain bullet IS an effective stopper. It is also quite accurate.
If you get yourself one of these historic pieces, I would suggest you change-out the front-sight blade and make and install a lower one which brings the muzzle UP about 16 inches on your 25-yard target. You do not mutilate the original blade and you DO keep it safe because it is original to the gun.
But you are now shooting a safe, dependable, reliable, inexpensive and ACCURATE piece which has a LOAD of combat history behind it.
For a cheap load which shoots well, try 2.1 (two-point-one) grains of Bullseye with a 158-grain SWC seated to the OAL of a Ball Mk. IIz round. If you run your own slugs, these cost about $3 a box to load up. That's .22 prices for something a LOT more fun. This is a safe load which groups well in all of mine: a Webley, a Smith and 3 different Enfields: Mark 1, Mark 1* and Mark 1**/1*.
Go have fun!