LOCKING SYSTEM PARADOX

For maximum strength the double gun must be fitted on the circle and therefore must have an action bridge, say some experts.

The recipe is a little cryptic, so we decipher it.

The action bridge is that bit in the middle of the side by side action body that joins the two sides. It looks like a little bridge, hence the name.

The rear lump of the barrel goes behind this bridge, while the front lump in front, obviously. Fitting on the circle means that the front part of the rear lump (gets confusing all this front and rear thing, the photos will clear it up, hopefully), will be in contact with the bridge when the gun is closed.

When the gun fires, there is flexing of the action. Most flexing is at the bottom of the standing breech. The black and white photo of the broken action body leave no doubt as to the location of stress concentration!

Side by side action broken along its most stressed part, the bottom of the standing breech.

Side by side action broken along its most stressed part, the bottom of the standing breech. Would fitting on the circle have prevented this?

By creating the bridge to rear lump contact this stress is managed via transfer to the barrel, say the experts and thus the barrel to action join is made stronger and more durable. It is a bad idea, say the experts, to have the firing stresses be borne by the cross pin alone, the bridge is there like a supporting act, so to speak.

YES, BUT...

Seeing the results of overstressing, and poor heat treatment that broke the action in the photo, I fail to see how any contact between bridge and rear lump would have prevented the mishap.

Moreover, it can be argued that by containing the stress, by trapping the barrels via "fitting on the circle", the problem is exacerbated. The bending stresses are kept within a small part of the action. And that small part is the most stressed part.

There are doubles, smooth as well as rifled, which have no middle bridge in their action bodies. The Chapuis is a fine example of such a system. There is no bridge, hence the bending of the action bar is free to develop along the full length of the action. The Chapuis is offered in some fierce rifle calibers, all the way to elephant gun power, with no ill effects.

Chapuis double. It has no action bridge.

Chapuis double. It has no action bridge. It comes in chamberings up to 9.3X74 that put five times as much stress on the action as a shotgun shell.

Reference must be made, yet again, to the Winchester Model 21 which was built with no middle bridge. In destruction tests carried out by Winchester the Model 21 outsurvived all its competitors, most of which had middle bridges and were presumably "fitted on the circle".

WHAT OF REPAIRS?

All doubles eventually wear and work loose. When that happens the usual repair is to fill the barrel hook, or use a larger diameter cross pin (hinge pin in American), or a mix of both methods. This process moves the barrels back, and they must be refitted to the breech face with a smoke lamp and all that highly skilled business.

But wait a minute! Moving the barrels back by definition deletes any contact between rear lump and bridge. The much touted "fitting on the circle" goes to pot. But the guns apparently still work OK.

HAMMER AND ANVIL

Yes, the double gun will flex back on firing. This we have been told countless times. But that is half the story.

If it bends back, obviously it returns to its original position. In effect the action of the side by side shotgun becomes an almighty spring and hammer during the firing cycle.

In phase one, on firing, pressure in the shell acts on the breech face pushing it back. The breech face retreats, by about 5/1000s of an inch said Gough Thomas, who was a highly qualified engineer and gun writer for the British Shooting Times.

The stress on the vertical breech face goes along the top surface of the action, elongating that side. There are also compressive forces acting on the bottom of the action bar, as well as on the opposite side. It is logical that if you stretch the right side you are compressing the left side a little.

And then comes the recovery. The action snaps back with great force. The elongation now becomes contraction. The evidence of this force is there to feel by running the hand along the top lip of the barrels. In most doubles that have fired a few hundred shots you can feel a raised lip of metal, material displaced by the hammering of the hard breech face on the softer barrel metal.

If the breech face is the hammer, during the recovery phase, and the barrels are the work, where is the anvil?

My view, and this is a personal opinion yet to be factually proven, is that the anvil is the cross pin. As the action contracts during the recovery phase the barrels are squeezed between the breech (hammer) and the cross pin (the anvil). The cross sectional area of these points of violent contact, (the cross pin, hook and the barrel breech ends), as well as their metallurgy determine the amount of deformation and possible looseness that might develop in a double. Friction wear by itself, resulting from many openings and closings, seems unlikely to be the only culprit in looseness. In my opinion, the repeated pounding on parts with insufficient surface area and/or hardness is more likely to cause looseness.

Additionally, it would seem that the squeeze put on the barrels by the recovering action body, will eventually overcome any impediments and develop fit and tolerances dictated by the forces acting on the material. No matter how good the "fit on the circle", it will be pounded to submission if it takes on the role of primary anvil. Either it, the rear lump and bridge, will be the anvil or they will be deformed and the cross pin will take over. That both should survive the pounding is unlikely. In my experience multiple bearing surfaces usually end up developing tolerances till the strongest points of contact take on the whole of the work.

The above also seems to apply to over unders. Large diameter trunnions, supported in the front by action body metal seem to outlast little trunnions. The presence or absence of side lugs seems to be secondary in the durability league. Proof here comes from the cheapest types of OU, the folding models which have a very simple lockup and no side lugs, and they seem to manage to survive rather well. And it is not only the cheap ones, the medium quality Verney Carron Sagittaire, offered in both smooth and rifled versions, has years of succesful service with no side lugs, not even bottom lugs. See some nice pics of the Sagittaire here:

http://www.chassons.com/encyclopedie/armes-de-chasse/Fusil_Sagittaire_Billebaude_Verney-Carron_cal-12mag.htm

Probably the archetypically heretic double gun design is the American L.C.Smith. It has a barrel hook and that is it, no rear lump, no underbolt. The relatively small, in comparison to lump dimensions, top extension is all the locking there is. The Smith sidelocks are super simple, using the single V spring to power tumbler and sear. Despite the simplicity, or perhaps because of it, the L.C.Smith has proven itself to be a durable shotgun that seldom works loose.

L.C.Smith action.

L.C.Smith action. Simple, without underlugs, yet strong and durable.

These and other thoughts go through my mind every time I watch highly skilled craftsmen fitting barrels to actions. Is so much expensive labor really necessary to be spent on that particular part? The usual answer is that this way of doing things is traditional. So what? Traditional is often wrong. For three thousand years we stored oil and wine in open topped urns. We now know that contact with air spoils both wine and oil, and we have turned to airtight lightproof containers. Modern tins are not as romantic, or as traditional looking as large earthenware urns, but using them sure beats eating rancid oil and drinking vintage vinegar!

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