notice some bolting systems have two locking lugs with a third top fastener, visible or hidden...
whereas some bolting systems have two locking and a bolt at the bottom of the action, with no third fastener...
why the third fastener? seems redundant...
The third fastener is to ostensibly prevent the standing breech from deflecting backwards under the load of firing. The joint between the action bar and standing breeches is under a giant bending moment when the gun is fired and is a weak spot in the action design. Poorly built guns, or those with incorrect heat treatment, can crack at this location.
The vast majority of third fasteners don't actually make contact in a spot that would allow them to bear load in this axis, so most of them are useless. Exceptions being the Rigby/Bissell rising bite, the Westley Richards bolted doll's head, and the Manufrance Ideal's version of the rising bite. Scott and Greener crossbolts are potentially load bearing but almost never fit properly to do so.
The two locking lug systems are actually just one lug - the bite nearest the hinge pin has such a small moment arm that it's effectively redundant. It serves only to make the gun a snap action by camming the bolt backwards (through means of impingement on the back of the front lug) as the gun shuts.
In general, a gun with a fitted third fastener of the correct design is superior to one without.
It was more common on rifles where a stronger method of fastening the barrel down and locked was necessary. For one, the third bite was farther from the hinge pin so it would be applying more pressure and if the third bite was made with a angle to it so the same angle was on the locking bolt it would always stay tight. The bolt/third bite would be self adjusting.
Ed, you might recall that our late friend Miller had told us of smoke checking actual contact of bolting surfaces in a number of guns with multiple bolting, and found that none showed full contact in all areas of bolting. In other words, only one or two were doing the work, and the rest were simply cosmetic.
A well designed shotgun can be adequately and safely bolted with a single bolt. The one I am most familiar with is the Syracuse Lefever, which does quite well with a simple single bolt engaging the top rib extension. Their biggest problem seems to be with top lever springs that become weak or broken, and then do not provide secure bolting. The bolts themselves are pretty simple, and fairly easily replaced when they wear to a point of poor engagement. Fox and L.C. Smith guns do pretty well with their single rotary bolts engaging the rib extension. But when fitted incorrectly, they are known to sometimes cam open when fired with heavy loads. Guns with a single well-fitted Greener type tapered cross bolt engaging the rib extension do very well too. I agree with Paul Harm that double rifles, which typically operate at much higher breech pressures, are more in need of well fitted and well designed multiple bolting systems. It takes a combination of good design, good materials, and careful fitting to achieve the most strength and durability. The guns that are prone to shoot loose and go off the face are the ones most likely to be deficient in one or more of these requirements. And of course, that sort of wear is accelerated by the use of loads heavier than the gun was designed to use.
thanks keith...
do you happen to know who invented the double under lug bolting system without top fastener nor rib extension?
best i can figure it was woodward of england...
That would be the Purdey double underbolt patent of 1863.
It was originally paired with the Purdey thumbhole underlever sitting inside a bifurcated trigger guard. Purdey then almost immediately formed an agreement with W&C Scott to use their spindle toplever opening system, in exchange for letting the latter use the new bolting system.
Keith, another thing to note is double underbolt guns being fitted on the circle (or lack thereof). I recently discovered that almost no shotguns were actually fitted to make contact on the draw, which undoubtably contributes to their going off face faster than they should - they're bearing the entire fore-aft forces from recoil just on the hinge pin. This is the case even with the British Best guns, much to my dismay.
There are very, very few manufacturers who properly fit their shotguns on the circle. Some Merkel guns have a little removeable insert of hardened steel in the draw and can be made to bear on the circle reasonable easily. Fratelli Rizzini guns are properly fitted.
Keith, another thing to note is double underbolt guns being fitted on the circle (or lack thereof). I recently discovered that almost no shotguns were actually fitted to make contact on the draw, which undoubtably contributes to their going off face faster than they should - they're bearing the entire fore-aft forces from recoil just on the hinge pin. This is the case even with the British Best guns, much to my dismay.
There are very, very few manufacturers who properly fit their shotguns on the circle. Some Merkel guns have a little removeable insert of hardened steel in the draw and can be made to bear on the circle reasonable easily. Fratelli Rizzini guns are properly fitted.
Kip,
The Baker Gun & Forging Co., starting with the Hammerless models in 1892 and also the Model 1897 Hammer gun gun(Syracuse and early Batavia), utilized a cross bolt in the barrel lug that engaged a cut in both sides of the frame. This in theory relieves stress on the hinge pin and holds the barrels tight to the breechface. This combined with the bolted barrel extension seems to make for a very strong lock up. Again, only if properly fitted.
John
so, besides purdey, who else in england, built hammerless side lock guns with bottom bolting only...and with no top rib extension or dolls head?
for example, did webley & scott?