Zwerge knows for sure what this is.
Looks like a WW1 era 303 to me
I agree with Steve, WW1 .303, I found a full clip of these in a friends grandads garage many years ago, all live, i dont know what happened to them, my dad might have thrown them in the river
PS, I would be careful with this round its live and they have a tendency to be a little unpredictable with such a large percussion cap at the base
Thats a Canadian 303 round, a Mark II, made by the Dominion Cartridge Company.
"Dominion Cartridge Company, military production was a simple D with a C-broad arrow for the Brownsburg plant which later became the Dominion Ammunition Division of Canadian Industries Ltd. They operated factories in Brownsburg, Quebec and Montreal, and the illistration is a sporting cartridge. The DOMINION headstamp was used on commercial ammunition from 1911 untikl 1955. They also produced .303 cartridges in... Ball, Cordite Mk 2, 4, 6 and 7."
Because this one has the brass casing it's not very difficult to wiggle out the bullet. I happen to have a hole drilled in my shed wall that I use to unload live amo. Stick the bullet in a hole, a wiggle it untill it comes loose. Pull the bullit out and remove the cordite. Than you can unload the cap in hte base, but I never bother about that. I simply glue the bullit back in and leave them as they are.