bansobaby
Accustomly titled
You can't legally shoot deer other than muntjac and CWD with .223 calibre in the UK.....Looks to me like a jacketed slug from a .223 (Remington) or 5.56mm (NATO) round.
Very standard hunting round if you're after bigger game like deer. If your timber came from woodland where there's hunting, finding slugs is unusual but not exactly rare.
We have an oak window sill which also had a slug in it but it was only found some years after installation when we were re-oiling all the windows