i know of this same thing happening several years ago from a friends incounter.
his young son got his 243 out of the unlocked safe and found a 39 cartridge on the top shelve.
the boy took it outside and made it go bang !!!!......the bullet did come out the barrel muzzle
but alot of jacket copper was left in the chamber.... it stripped the outer jacket right off !!!
so it can happen.... i seen the aftermath shortly after the incident.
luckly the rifle was a ruger, a strong action and very well build. after a complete disassembly
of the action, scoped the chamber and bore to find no damage at all !!!
reassembled and fired in the test cell over 25 shots. no different performance.
today this rifle is still being used.
it can happen....