doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com gap between barrel flats and water table?

I am tempted to remove the barrel lock on a M-21 and a L. C. Smith (both 12 ga.) and shoot. Also, might ask an Eng-student who shoots, about stress/strain gauges. BTW, for those who think a M-21 is very,very long, my M-21 is 2 5/8 from face to center of hinge pin; so is my L. C.(both 12 ga.). I will also check " The Elastic Strength of Guns" by Phillip R. Alger Professor of Mathematics USNA to see if I can find a math way to calculate the amount of flex at 8000 psi.

Slightly different theme here is that a B25 [and similar guns[ with little or no bite will shoot bottom barrel OK but has a tendency to come open when top barrel is fired . All to do with the height of the back pressure above the point of bolting , I'll let others argue the technicalities .As a result top fastening guns will require less force/pressure on the bolting to keep them closed than bottom bolting guns .To further add to the discussion you would also need to factor in the length of the bar , depth of lump and height of the fence .
What ever this is there is still a need for a gape between action flats and barrel be it on a side by side or an OU. Webley reckoned the ideal was .002/003" at the front and .004/6" at the back. The gape was left slightly bigger on jointing to allow "blacking down" after hardening when the gun was freed.

Craig Libhart,

There is whole bunch of clips on Youtube showing interesting phenomena with slam fire shotguns as well as firing a shell in an open 12g barrel using a bb gun as a firing mechanism.

In the bb case it was interesting to see the barrel recoil even in the absence of any lockup.

In doing these BB fired tests did they by chance conduct any with the barrel secured from recoiling with the shell & if so what occurred. If the barrell recoiled with the shell this simply means the inertial resistance of the barrel against moving was less than the frictional resistance of the shell to chamber. This "Most Certainly" does not ensure that if the barrell were given more resistance against moving the shell would remain firmly affixed in the chamber.
In support of the fact there is indeed back thrust to the shell head is the number of cracked double frames which have occurred over the years as well as certain Pumps which have "Earned" a bad reputation due to a weak bolting design.
While not shotshells no reasonable person can deny that the self loading action of the .22LR, .25ACP, .32ACP, .380ACP, 9mm Makarov, a few 9mm Parabellums & .45ACP sub machine guns etc is brought about purely & entirely from the back thrust of the cartridge.

Craig:

An 'old goat' story for you... even after all these years I still chuckle when I recall the scene.

A shooter at a trap club I used to frequent long ago and far away, had just attended a class for 'home defense firearms instructors' and was eager to show off his new found expertise (what could go wrong?).

The guy was a mountain of a man who wore a belt buckle the size of a hubcap, and a cowboy hat to match. Being of considerable wealth, the hat was obviously quite expensive - a gen-u-ine Stetson.

He had acquired at the class a machined gizmo that consisted of a shell holder and a spring loaded firing pin. A 'zip gun' sans chamber.

The purpose of this gizmo was to fire the primer of a shot shell to demonstrate that small arms ammunition was safe to store in the home because it needs a chamber to build pressure. So far so good. Old Joe claimed to have conducted the demonstration many times and was eager to do so for us - an assembled audience of perhaps 20, some of whom raised objections to the demonstration being conducted right there in the clubhouse at the poker table.

Safety minded as we all are, we adjourned the discussion to the porch where Joe inserted a brand new Federal Champion trap load into his gizmo. Cocked and loaded he mentioned that he should have a container of some sort to catch the shot as it harmlessly fell from the about to be fired shell, apparently so as not to spill shot all over our pretty porch.

Whereupon he grabbed his Stetson off his head, held it under the shell, called 'Pull' for effect, and tripped the firing pin.

The demonstration was a success... sort of. At the primer firing, a small cloud of that perfumed Federal smoke went up, and as promised the shot and wad fell into Joe's hat. However, so did a rather considerable amount of ignited powder and Joe was now holding a burning $400 hat.

Joe then did a very interesting dance, swinging the burning hat around attempting to extinguish the flames. A man that size needs room to do such work, so we gave Joe and his flaming hat a wide berth.

That day alone was worth the many years of dues I paid at that club!

SJ:
Thanks for that confirmation. The guy who told me about the M12 is something of a BS'r so I didn't know what to think, but if you witnessed something similar then it must hold water.

Still am intrigued with the notion of a SxS being fired without a factory lock up. Hmmmmm...

A little like driving a car with no breaks > It might be OK >

2Piper,

The barrels, i think a Mossberg pump barrel, in the BB demo is tied to a bunch of thick tree branches, and in recoil drags them back a fair distance.

I have seen and personally photographed a few receiver crackings in SXS doubles. I have yet to see an OU cracking its receiver.

Again, there is the total lack of reference to the Poisson effect, (the radial expansion and axial contraction, and RECOVERY of the chamber part of the barrels), in the various analyses I have read. And how the Poisson effect may contribute to these frame crackings.

I do not discount the pressure of the cartridge on the breech face, but is it the only part exerting pressure on the breech face? And what happens when the frame recovers from its flexing and it in turn squeezes the barrels betwen breech face and cross pin? It would seem that flexure recovery turns the barrels into a piece of "work" caught between breech face and cross pin and this may be happening when the barrels are recovering from their momentary expansion-contraction.

The strength of the locking system in a break open gun seems to be secondary to the other elements contributing to the strength of the action. Other "bits" seem to be more directly involved.