Justin, I remember text and illustrations from a couple of sources (think maybe Vol. 1 of The Modern Shotgun and in one of Garwood's books) indicating that all locking bites located top of breech (not only Greener and Kersten but also rotaries and even non-locking doll's heads) have the advantage over underbites of being further from the hingepin, allowing them to act thru a longer lever arm against rotation about said pin. Whether they are battered to a state of distension and deformity faster than are underlugs and sliding lockbolts I have no idea. The longer lever arm applies to both the translation of the opening movement around the hingepin to the lockbolt as well as its restraint by that bolt; would there be potential for a larger battering movement but a milder force applied than would be the case for bottom bolting?

It has been suggested that accurate fitting of crossbolt and underbite[s] to share the load applied on firing is difficult of accomplishment. I tend to believe that an inexpensive gun with multiple locking (Noble Arms m410) might well have a Greener which is more for show and not much for go. I can see no justification for redundancy of underbites unless they share the mechanical load equally. Perhaps Purdys are fitted with rigorous patience and each pair of bolt and lug takes a share of the load right out of the box. Again, using the example of a Spanish gun made to a low price, I would assume that one bolt waits its turn on the other which is a closer fit initially. I think they would tend to work in concert (dividing the load) after the tighter fitting of the two is "battered" to a tolerance of movement equivalent to that of the other as fitted.

My recently acquired Merkel from the late sixties has only the Kersten X-bolt. Even the Germans are able to sacrifice painstaking to a price point. Notice they stuck with the top bolting rather than the double underlugs. I don't know if that would have been a decision based entirely on economy of manufacture or if the company reputation for quality and longevity was served well enuf by the Kersten alone.

I've always wondered about the efficiency of floorplate-piercing lugs (recoil lugs). Do they share the work? How much and for how long?

jack