Peter and company,

Thanks for the replies on this subject. You confirmed what I had first thought that it's not possible to go off like that.

Would a pierced primer do similar damage ?

Just to clarify, the remainder of the rounds in the magazine did NOT detonate, but the OOB discharge caused the remaining rounds to exit the magazine through the floorplate and land in front of the gun.

My suspicion is pointing to the ammunition. Old 1950's rubbish which is known for hangfires and slow ignition.

The set armorer in question is a US Army veteran armorer, who is a pretty decent bloke with guns, if anything can be said his only fault is lack of knowledge on older Britishicon Weapons (which is why they asked me to come out, unfortunately after the fact)

The studio services that rent the weapons are utterly clueless. They butcher and bastardize weapons to make them serviceable to fire mostly blanks, so when it's really needed to fire live rounds, they don't necessarily work 100% reliably.

Don't get me started to what they did to the poor Vickers Gun in this weeks shoot............