Eeeek! Sorry to hear this.
I suppose I've been lucky and never had anything like this happen, so not sure really as to the best way other than a full strip so you could (very, very carefully), using a rod tap them out. The full strip meaning you can knock them out from breech to muzzle; not sure how practical it might be to try and do it the other way around?
The full strip would also allow a look inside and get to the cause in the first place......piston and / or breech seal in need of replacement?
Hope you get it sorted soon and someone else be along shortly with more experience in such matters.