Please forgive the lengthy (and at times elementary) post - likely full of typos and rambling. I have to admit that I’ve not looked at assembly drawings beyond part id so maybe I’m way out of line or totally off base. I am also no gunsmith nor do I claim to be. That is to say - this is only what my mind’s eye shows when I think of how a box lock works. I would greatly appreciate further understanding through correction from those that know.

To anyone kind enough to read this through step by step, where am I mistaken?

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Ignoring gravity. Lay a loaded barrel on a flat and the barrel face back against a vertical surface (breech) so that all gasses are contained and do not escape out the rear when a cartridge is fired. No lock or hinge engagement at all.

Pull the trigger - the action (frame) propels rearward and the barrels propel forward. The surfaces separate. (barrel face and breech) Velocity of the cartridge (as intended) is low because gas escapes.

Add a hinge - pull the trigger. Action propels rearward and barrel movement forward is disrupted by the hinge. The energy is now violent because of the hinge’s sudden disruption of barrel motion and compounded further by the spike in velocity due to the partial containment of gas. The energy has nowhere to go so it’s directed upward toward the path of least resistance. The barrel separates at the top of the breech while the base of the barrel remains tight at the breech base. The barrel folds downward about the fulcrum created by the hinge.

Add the under-bolt - pull the trigger. The bolt controls upward barrel movement therefore, preventing the barrels from folding, to an extent. The stack-up of tolerance between the bolt and the bolts channel in the frame equals the distance that the barrels will move upward under load. When this occurs, the barrel will separate from the breech at the top but remain closed at the bottom due to the hinge that is preventing forward movement. (the fulcrum). Velocity skyrockets.

As the bolt and bolt channel begin to wear, the stack-up of tolerances begins to open up as clearance between mating parts increases. Therefore, upward movement also increases, allowing for higher separation between barrel and breech at the top while the bottom joint remains tight. (assuming hinge and barrel hook has not worn)

The dolls-head barrel extension intends to hold the barrel up against the breech in a straight line in unison with the hinge under load. Therefore, containing all energies and forcing them in one direction, backward, while at the same time preserving all gas for cartridge velocity. The barrel and action are now one pc because the breech is captured by the barrels at the extension up top, the bolt at the bottom, and the hinge pin.

As I understand, this helped but it was flawed because it didn’t fully account for the store of energy channeled to the hinge creating a fulcrum which would still send energy upward. Hence, the cross-bolt. This combination of locks, now contains ALL of the energy so it is now directed rearward under load. The stack-up of tolerances becomes a virtual non-issue because geometry of the various locks lends itself to containment of all movement, inherently. Barrel separation at the top of the breech under load becomes virtually non-existent. Further, the cross-bolt now limits the heavy lifting required by the underbolt because when clearances max out, the cross-bolt limits further upward movement which substantially reduces strain against the underbolt and, the barrel lug for that matter. This relieves the brunt of energy that the underbolt would otherwise have to handle if the cross-bolt wasn’t there to limit travel. The underbolt will eventually wear, maybe, but it will only wear from friction. The cross-bolt also benefits the underbolts channel in the frame by offering protection of the no-tolerance fit that the machinist worked carefully to achieve while building the action. The protection of the no-tolerance fit also inherently offers a great deal of protection to the frame itself which is case-hardened so that it resists frictional wear between bolt and channel, yet the core of the frames material remains ductile instead of being inherently brittle and prone to fracture as a result of, strain.

Some manufacturers dropped the cross bolt and incorporated a bite at the top lever thinking it more efficient.

Parker did something different. Parkers approach was very clever, dare I say superior to the cross bolt as it relates to longevity of the entire mechanism despite potentially higher maintenance requirements.

Dean says otherwise (and my reverence for his knowledge would not allow me to dispute his assertion). However, from a mechanical viewpoint, assuming Parker incorporated the extension into their design for strength and function (i.e., ejector guides / stop plate).

Parker added the removable wedge. That’s genius in its simplicity because that wedge is designed to wear and it’s a sacrificial item. I can only guess that it is intended to be softer (lesser heat treat) than the underbolt that engages it. As the wedge wears, the user will notice that the lever is approaching center. It’s common knowledge that once the lever approaches left, the gun needs attention. What also happens though is the front face of the barrel extension will begin to wear as movement of the works increases under load. If the wedge is replaced before movement under load increases, the only wear to the extensions bearing surface will occur as the action is opened and closed by the guns owner. That wedge assures protection of the extensions bearing surface and negates the necessity of a cross-bolt. The owner also gets the benefit of longevity of the guns inner works because all movement under load is controlled at all times. Movement in a mechanism that is not controlled allows for rapid wear because wear is not linear once it begins. At any time, the wedge can be replaced and voila -a tight gun. Great design.

John Browning - John Browning saw something different. John Browning removed the hinge and captured the energy. Pure_ genius.

All that said, the well-established expertise that the double-smiths who determined that the extension or bolt, unnecessary; I would guess, totally correct however that does not negate the engineering principal that is totally correct. Or, I’m missing something altogether.
Thank you, John for sharing your wisdom. Also, thank you Dean for pointing it out as a practical matter.

Please feel free to comment.

I think a possible flaw in your thinking is that I don't believe the barrels are "propelled forward". The breech would go backwards, resulting in a separation (assuming the actions is not constrained by your shoulder as an example), but I don't understand what would propel the barrels forward.

Wouldn't the force of the breech going backward also push the barrel forward? Maybe not one to one, but forward nonetheless?

Not in the physics classes I took, but that was 50 years ago, and things may have changed

lol..touche'. So if you mounted a gun up to your shoulder as a back-stop, the barrel would remain stationary (figuratively speaking) as your shoulder stops rearward movement of the frame once the gun is fired? Don't misunderstand, I'm not suggesting that the cartridge carries the barrel like a projectile, of course.

I suppose that after the breech separates from the barrels as it moves rearward, that the escaping gases going out between the breech and the barrels could exert some very small forward pressure on the barrels, but i would judge that to be minimal

Physics says that "for every action there is an equal and opposite reaction."


I would also add that the angle between the breech face and the action flats (water table) is ever so slightly more acute than a perfect 90 degrees. This would change a lot of people's thinking of an upward force upon discharge.





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Exactly correct.

I did a poor job of laying out the fact that my logic intends a definite stop of rearward movement as the barrels are being carried (by the hinge) backward. Once it hits stop, force reverses direction and that action gives rise to an equal, reaction (forward barrel motion as the frame remains in a fixed position 10lbs rearward>stop>10lbs forward) I wasn't 100% certain of the exact 1 to 1 reaction though b/c of gasses

The standing breech not at ninety-degrees is something I hadn't even considered.

Just had a look - thank you Dean :bowdown:

How in the world did I miss that - 'tis why I'm willing to put myself out there and look like a j-ass.

The classically trained double gunsmiths have it right and I am just flat wrong :duck:

Back to the idea of dropping the barrels down by "grinding the flats". This will simply
Not work because then you are moving the center point of the barrels down which, if done enough, can cause the firing pins to not contact the primers correctly resulting in misfires.

This whole subject is simply the the matter of all of these guns being individually hand fitted and the amount of variattion that exists from gun to gun. It is simply the nature of the beast that sometimes can be reasonably worked through and somtimes not.