Regarding your comment of the Greener cross-bolt being second strongest,no?
I disagree - I think the Greener cross bolt is not a good system as once it wears (and you see alot of them worn), they come loose and then shoot even looser as time goes by.
I have seen alot of loose Greener Shotguns.
I prefer the Webley PHV 1 or Screw Grip system which continually seem to lock up the action even as it wears - which is due in no part to how it is designed.
Just my HO. Someone like 400 could probably give a better technical explanation.