The actor plays a military veteran in the film, which depicts the 2012 terrorist attacks on a U.S. compound in Benghazi, Libya, and he received weapons training from real members of the U.S. special forces.

However, John was left with a painful reminder of his first on-set gun battle as he was hit by a red-hot shell casing which stuck to his neck and left a nasty burn.

"The first day of shooting we all lined up and started shooting guns for the first time and... one of (the shell casings) flipped up and hit me in the neck which everybody was like 'Hot brass it happens'," he tells U.K. chat show host Jonathan Ross. "(Hot brass) sounds like a cool jazz band and then it hits your neck and you're like 'This is not a cool jazz band, this is awful'.

"I had sunscreen on so the casing stayed on my neck and the rule the Navy SEALs taught us was you can never stop firing on a firing line... So for about a minute (the shell casing) was searing my skin and I still have a scar... so now I'm a real man!"

John's appearance on The Jonathan Ross Show is due to air in the U.K. on Saturday (23Jan16), and the film is set for release in Britain on 29 January (16).