NOT FITTED ON THE CIRCLE
This Midland Gun Company double looks like any other round bar back action hammer gun.
Traditional shotgun lore insists on fitting on the circle for a strong and durable joint between barrels and action.
Fitting on the circle is a simple concept, but hard and tedious to achieve. It involves full contact between the front facing surface of the rear lump and the action bridge, that bit of metal spanning the the middle of a side by side action bar. To get these two surfaces to touch when the action is closed requires lots of smoke fitting of the lump, careful and precise filing and therefore adds cost. The time of skilled fitters is expensive.
The barrels have a solid lump with no central gap.
What happens when there is no central bridge in the action bar and no separate rear lump? The question is answered by the low cost Midland Hammer Gun in the photos. The barrels have a solid one piece lump, the action has a single long slot with no bridge. This is an old gun, its estimated manufacure is somewhere in the 1930s and close examination shows it lasted fairly well. The type of use it has seen this is attested by the state of the stock. Judging by the nicks, gouges, dings and scratches this gun has seen a lot of not so gentle use, yet the action held despite it not being fitted on the circle.
Also interesting are the proof marks on this humble shotgun. It was made with 2 3/4 inch chambers and proofed for 1 1/4 oz shot charges. In its day these were regarded as heavy loads, equivαlent to today’s baby magnums. Greener calls the 1 1/4 oz load a duck load.
The action has no central bridge.
Expert opinion says that in the absence of fitting on the circle the brunt of the forces on firing is borne by the cross pin (hinge pin). If there is a mid bridge and fitting of the lump then these forces are borne partly by the bridge, thus alleviating some of the strain on the cross pin.
The fact is that all break open shotguns will work loose at some point in their career. Repairing them involves fitting a larger cross pin, which pushes the barrels back a little. This push back must by definition nullify any fitting on the circle that might have existed before the repair, yet the repaired shotguns seem to have no problem.
Proof was for 1 1/4 oz shot with original 2 3/4 chambering.
In the Midland there are signs of a cross pin replacement, as revealed by the polishing of the front of the action. Looking at its simple one piece lump it is evident that the repair must have been an easy job with no elaborate fitting involved. It is equally evident that the original fitting of the barrels was an easy and fast business. As a guesstimate I would say that it took about one tenth of the time to fit these barrels than it took to do a full fitting on the circle. Careful miking of the lump shows that it is a few thousandths of an inch thinner in the back, thus allowing for the torsional flexing of a side by side action. The mike showed 8.88mm in the front half of the lump and 8.55 in the back half. Whoever fitted this lump knew what he was doing. But what he did took little time and did not cost a lot.
Dings and gouges in the stock show a lifetime of hard use.
This is not the only side by side with no central bridge and no fitting on the circle. The legendary Winchester Model 21, the Beretta Vitoria hammer gun, the Bernardelli folding Game model, are examples of doubles with no central bridge. Incidentally they are durable shotguns with no reputation for any tendency to shoot loose prematurely.
Perhaps it is time to re-examine this fitting on the circle assumption a bit more analytically.