"Try standing the rifle on the butt, and shoving the barrel down forcefully."
That might work but, even if I KNEW it was on an empty shell it would make my skin crawl; I would put a book or magazine on a board outside to protect the muzzle and use body weight to push the butt stock down enough to free the bolt and lock it back. That long magazine spring and brass friction ring can be pretty resistant to compression, I'd remove the forend and make sure the flat end of the friction ring faced the barrel band before starting.
Do NOT let a locked back bolt slam home without the barrel and recoil spring system being securely in place!