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.
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