I was reading a book about the Thompson SMG. It referred to it's rounds as .45 APC. What does APC stand for?
And there was me thinking this would be a thread about Armoured Personnel Carriers... As Martin said, ACP - Automatic Colt Pistol - from the fact the round was originally designed for use in the new (and now near-legendary) Colt M1911 semi-auto pistol.