Now that was super quick…
Not sure on the size but it is a signal flare.
Your turn.
Nope not a shotgun shell.
Wombat has it, it is a signal flare.
I’m after some if anyone has any.
That’s a bit deceptive, it could be a pistol/rifle round or an artillery round.
None of those.
These used them - 
Ahhh got it, 
Still no one???
It’s not a .22 or Rimfire…
Would it be a English Electric Brass Engine Starter Cartridge/Shell?
Why yes it is.
They used three of them in each jet engine to start them 80mm Diameter 185mm high, full of Cordite.
That would have been a bang and a half with all 3 going off.
Is it a .45?
358 Winchester or 35 Remington
Is it a rim fire?