"Automatic single shot" ,,,, I bought a NEF 45-70 several years ago. First 2 shots it did the same thing ,recoiled ,poped open,ejected..... promptly took it in the house and started to troubleshoot. Factory 405 remingtons, almost new gun (the 2 boxes of shells that came with it were missing only about 8 rounds!) ,no aparent "gunsmithing". What I found was that the locking lug swings to engage the cut in the barrel lug. It was dragging on burrs and only engaging about 25%. It was enough to allow it to fire but if the gun was held solidly into the shoulder and left hand controled the recoil as much as possibile then the locking lug disengaged due to inertia as the gun stopped in recoil. I shot it again letting it recoil more freely and it did not open. Stoned the burrs off and all is good. Probably would have worn it but it was EXTREMELY disconcerting to have the gun open at the end of the recoil impulse. By he way I still have it,I really like it but will NEVER fire it off a benchrest. About 6# 45-70 is OK from field rests, sitting .standing ECT ,Cant imiagine having that thing come sailing back at me with a typical loose BR hold!
As well as I like the NEF, it's just to big and "industrial" for a rook and rabbit rifle. I've looked far and wide for one I could afford. When I could afford it ,it just didn't "talk to me". I settled on the american version of an elegant small game centerfire from an early era. My 32-20 model 25 fits the bill bor me. I still watch for English single shots just in case cost and appeal coincide!
Sorry for the long wandering post ,I'm a bit ADD