WHERE IS THE LOCK UP?

It has been called Palintid, guerrilla gun, slam fire, zip gun. It is a crude setup consisting of two pipes contrived to fire a shotgun cartridge, usually 12 gauge. See it here before we proceed:

As you can see the Palintid is an open system. There is no locking. Yet it is evident from this and other videos of the type, that it holds together on firing. A quick reference to another type of gun, the blow back auto will shed some light on what holds the crude Palintid together.

The blow back system has no locking either. A relatively heavy breech block, backed by a stout spring, presses the cartridge in the chamber. On firing the powder ingnites exerting pressure in all directions thus driving the bullet to towards the muzzle. The same pressure also presses the cartridge walls against the chamber and the cartridge base against the bolt.

It is interesting to note that early blow back arms had problems with cases sticking to chamber walls. Special grooves were sometimes cut into the chamber to allow powder gases to envelope the cartridge case, equalise internal and external pressure, and thus prevent sticking. From this fact it would seem reasonable to infer that the case sticks to the walls of the crude Palintid chamber. Presumably the chambers are rough and the roughness aids in the grip of the outer cartridge walls to the chamber. Once the case expands and seals the breech it turns the barrel and the shell into a unitary mass. The outer pipe is there as a support to the cartridge base. As the inner barrel and cartridge recoil in unison they meet the outer pipe and all three recoil together.

AND IN MODERN SHOTGUNS?

Inevitably the example of the Palintid and the ability of these crude devices to contain and manage shell pressures must raise the question of what happens in a modern shotgun.

For many years the mantra was that a modern shotgun needs a solid lock up to contain pressures. It has been said by qualified people that the cartridge head delivers a "hammer blow" to the breech. Yet the Palintid shows that the cartridge, even in an open and unlocked system, does not act alone but in unison with the barrel. If a hammer blow is indeed delivered to the breech face, then it is probably delivered by the barrels and shell acting together and not just by the shell head alone. Corroboration of this is the obvious sticking of the case in the chamber and the need for strong levers in the form of extractors to get it unstuck. If you ever had an extractor ride over a shell you will know how strong the bond can be.

Additionally there is the Poisson effect of thick tube radial expansion and contraction to consider. The chambers of a shotgun qualify as thick tubes since their walls are more than one tenth of their radius. As the chamber area of a barrel expands during firing, there is a proportional lengthwise contraction. The contraction is tiny in measurement, but even so it must contribute to the overall dynamics of the situation. And at this point it is worth risking some speculation as to what happens during the ignition phase. The emphasis is on speculation.

The gases expand, acting in all directions. The shell walls expand and stick to the chamber. The wad is driven forward. Barrel and shell together move back pressing against the breech face. The chamber area of the barrel expands radially and also contracts lengthwise a tiny bit. The breech face flexes back. It is an open question as to whether the pressure on the cross pin (hinge pin) is decreased in this phase.

Then comes the recovery when the breech face springs back, the barrel recovers and contracts radially and stretches lengthwise back to its original length. Arguably during this phase there is pressure on the cross pin, both from the recovery of the action body and the lengthwise return of the chamber area to its original dimensions.

It would be interesting to see some high speed photography of the barrels and action of a double shotgun during firing, to see what really happens. Unfortunately those that do high speed photography devote their skills to photographing projectiles coming out of muzzles rather than on the action end of the shotgun. High speed shots of the action would enhance our understanding and lead to safer designs. They will also solve the long standing speculation as to what happens to an action during firing. The only thing we need is a willing photographer and a pile of money because high speed photography is a high cost pursuit.

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