We found this in our garage in the uk any ideas what it is
Not my area of knowledge.
It might be a 30 x 170 KCB. Possibly a TP (Tracer Practice) loading, and maybe still live. If you have any doubt, report it to the police.
35 x 228mm Oerlikon
BM: British Manufacturing And Research Company.
18: Batch number.
84: 1984 year of manufacture.
From 1971 to May 1988, British Manufacturing and Research Company Limited (BMARC) was the UK-based ordnance and ammunition manufacturing subsidiary of Oerlikon of Switzerland.