Some feedback from the person that has found these,
“Thanks again for the input but I’m sure they are not .380 bullets that have suffered from general corrosion, to answer the various questions.
They definitely still have their metal jackets, the couple of copper jacketed ones came out bright copper coloured although they quickly dull once the air got to them, the majority are presumably cupro nickel and have a reasonable silver appearance which darkened over a couple of days. When you look at the base of them you clearly see the difference between the jacket and the core.
I would agree with you that on first looks they appear to be Mk2 .380”, the profile is very similar, they have 2 rings and they come in both copper and cupro nickel jackets. However I’ve found more than 20 of these that are in reasonably “good” intact condition and few other which are mangled after hitting something. The good ones are consistently the same weight and diameter, there is very little variation in individual bullets and across the group (0.2grams/3grains) which I would expect to see if general corrosion was the explanation. Also I don’t see signs of level of corrosion required to reduce the diameter and weight significantly, yes they are lightly pitted but the features (rings, arrow stamps) are still quite sharp.”
Could they be intended for the .340 Tranter revolver?
Was there any military use of the Tranter?
Could they be for the experimental .34B BSA pistol?
The only picture I found of a .34B BSA pistol cartridge has a very different bullet profile.