Can anyone ID this round?

[rockquarry]

I remember reading something about the British Military tests of the Mars pistol. In the final test report, there was a comment made to the effect that “Those who fired it once during the test could not be persuaded to fire it again.”

The pistols were huge and probably very heavy, but the cartridges were pretty high performance, even by today's standards.

 

[AJ]

Try these folks and report what they say. International Ammunition Association Web Forum

 

[Kp321]

It looks like a 45 ACP shot shell that was issued for survival kits that someone stuck a bullet into.