Thoughts on the Trapdoor action

[Jedman]

I recently bought a M 1884 trapdoor carbine and I also own H&R officers model and a Calvary carbine modern replicas. Checking over the original I studied the locking and how the breech thrust is supported by the trapdoor and the receiver. Reading the thoughts of Frank de Haas , he says the close fit of the trapdoor to the receiver supports the thrust and the hinge pin does nothing but provides the pivot for the trapdoor.
I started with the original M 1884 and with the action closed and locked I can get a .022 feeler gauge between the receiver face and the back contact area of the trapdoor bolt and this action is not worn out or in poor condition. So being curious of how my H&R replica rifles measured I was surprised to find them both almost Identical in clearance maybe .001 tighter and they have been fired very little.

So what Frank de Haas says about the close contact between the bolt and the receiver face is not true. I did not try to measure headspace on any of these guns but would guess even if it’s only .004- .005 then when they are fired there is a lot of space that has to close up for the breech thrust to be supported. Even thinking about the slight angle of the rear of the bolt and the mating surface of the receiver the bolt would want to slide rearward and travel upward except for the close fit of the locking cam that keeps the rear end latched.

For the people who own and shoot trapdoors and study their actions , how do you feel the breech thrust is supported ? I like the actions design, especially at the front of the trapdoor how it is wedged under the upper hinge supports on both sides that are part of the receiver and how the ejector/extractor works. It must be quite strong and rugged to hold up shooting a large cartridge such as the 45-70.

From what I have read about where the action will fail if severely overloaded is at the receiver ring where it is drilled for the ejector spring and plunger. That tells me the hinge pin isn’t taking much breech thrust and the rest of the action is quite strong 💪. What are your thoughts ?

Jedman

[cwtebay]

Following

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[Deadeye Bly]

The door on a trapdoor is locked in place by the cam latch. This cam transfers the backthrust upon firing onto the breech plug. The hinge pin takes no thrust, it can actually be fired with the pin removed as the door locks in place under the ears on the receiver. I would not recommend this though. The door does not make contact with the back of the receiver, only the cam with the breech plug. While not an overly strong action by design it is plenty strong enough for black powder loads in the 45/70 cartridge.

[Jedman]

Attachment 284381Looking at this sectioned drawing of the action it looks to me that the point that the locking cam is in contact with the breech plug is above center of the stem that it turns on and any rearward thrust would cause it to compress the cam spring. But in looking at the cam on my rifle it is shaped differently than this drawing and with the trapdoor open and pressing on the rear cam face with my thumb it doesn’t compress the spring and is more directly inline with the stems centerline.
I did a lot of online searching for a explanation of how the trapdoor action supports the breech thrust and found virtually nothing . You may be correct but it does seem a unusual way to support the trapdoor ( breech block) ?

Thinking of the way the Martini actions breech block is supported by the rear contact with the receiver and the pivot pin is a split pin that can give slightly without bending makes sense that the thrust is not on the pivot pin.
So if the locking cam is what takes the thrust it is transferred to the locking cam stem which is actually about the same diameter as the front hinge pin.

Jedman

[Drydock]

The forward cam surface is in contact with the interior surface of the block. Thrust is transmitted from the block to the cam thence to the tang recess. The geometry of the cam forces the block downward into the trap during firing. The front of the block is retained from upward rotation by the ears either side of the hinge. Neither the hinge nor the cam shaft take any load. Removing the cam you will see burnishing on the forward surface of the cam and inside the cam recess showing the thrust path. The drawing is a poor one, showing incorrect cam placement and excessive clearance around the cam.

Though not a "Magnum" action, it is rather stronger than some might think, thanks to the vectoring of the thrust forcing the block downward into the trap upon firing.

[Jedman]

Thanks Drydock, Your explanation is very sensible. The picture of this action that I posted I can see is wrong.
Looking at the picture it shows clearance on the front side of the cam with the breech closed and locked which has to be wrong also. Thank you !

Jedman