My mistake on the calibre, it is a .22
There is no sign that this was a mod, the bluing looks original. I don't think this is damage caused by over power. I feel it was purpose done.
Rapidshot's idea seems the best possibility to me so far, I do remember seeing gas converted (home made) projects turning up at Bisley many years ago.
Unfortunately this was part of a deceased estate and there is no one to ask.
I wondered if could have shot some form of spread shot, but clutching at straws.....