Barrel lockup question

[hogrider04]

I've hunted all over for an answer to this, but couldn't find anything definitive.

Using the Trail All Weather as an example, which lockup is correct and would be the most stable??

1. Contact between the top of the breech face and the compression chamber, with it being held in place with the alive jam under the detent.

or

2. Contact with the underneath edge of the breech face to the detent pin and the alive jam under the detent. There would be no contact between the breech face and compression chamber other than the seal.

Mine is currently like option one, but I'm wondering if two wouldn't be better. Your opinions please.

[Motorhead]

Well ... If my memory serves me correctly in this instance .... Rifle in question has NO ledge or indexing shoulder within the breech pivot mechanics.
So ... What bottoms out is the actual face of barrel / breech against face of main tube / transfer hole area.  If the seal ring is excessive in height it gets compressed to a point where it's yield of compression gets maxed and it holds back metal to metal contact making for a spongy inaccurate lock up.  Seal just right or too thin metal on metal 

High end barrel breaks have a mechanical ledge / shelf that bottoms out upon breech closing having a fixed gap present between barrel & main tube of which the breech seal bridges the gap for a uniform seal. 

The inexpensive guns just use an over travel cam/ramp lock up that simply forces the two parts together upon closing.

[robtmc]

Thanks for the description of lockup designs Scott.  I asked the same things months back while trying to track down the source of my Trail's erratic shooting.  After some reflection, it was logical that the crude retention mechanism was only good for closing tension, not indexing.

To the OP, I did breech seal shims just to see what happened, and was immediately in a whirlwind of vertically strung shots that drifted as the seal compressed.  I went back to the previous (non-leaking) fitment and all was well.  Metal-to-metal contact was obviously the only way this design would be consistent.

After some finish and flattening metal work on the breech face to square it up, the rifle now shows a nice, even wear pattern across the mating surfaces.  Accuracy has gone way up since then as well.  Before, the only contact appeared to be at the top left corner of the breech.

[hogrider04]

Actually robtmc, your post was one I read that got me to looking at this issue. Did you ever check to see if the breech bottomed out on the detent pin after you did the face work? It sure looks to me that it would if the breech face was milled correctly.

[hogrider04]

Thanks for the response Motorhead. Up until today that is exactly what I thought, but there is a ledge/shelf milled back just below the breech face that appears to sit just on top of the detent pin when locked. It's hard to tell when the rifle is assembled, but it sure looks like it should/could set on top of the pin. It looks close enough that it inspired my curiosity. That's why I posted. the contact between the top of the breech face and compression chamber looks like it could change as  it wears in whereas the ledge/ shelf contact with the detent looks like it would be a little more stable. Anyway, just got me curious as to where the contact point should be.

[robtmc]

Actually robtmc, your post was one I read that got me to looking at this issue. Did you ever check to see if the breech bottomed out on the detent pin after you did the face work? It sure looks to me that it would if the breech face was milled correctly.

I did not remove enough metal to change the amount of movement in barrel lockup to affect that detent pin clearance.  Just wanted the breech to contact as evenly as possible.  The only way to see how close that ledge comes to contact is to put some soft material in there and measure compression.  Did not bother after my experiment with shims.

You can't get in there to measure how close the ledge and detent are and I sure was not about to modify things to see what happened.  You could end up with the mother of all inverse barrel droops! I shimmed enough to see the slightest gap in the breech and it was a joke, sure was no solid mechanical lockup happening. 

The older Crosmans had an opposing "jam" chisel rather than the detent pin.  That would "seem" to be just another variation on the same theme, not a true indexing lockup.

[Motorhead]

As too some rifles having the cross pin / ledge stop arraignment,  pending how gentle or rough operator is in closing action has a huge effect on how long the mating surface stay useable.  Slamming the barrel closed places EXTREME loads on the interface hammering the pin and ledge surfaces ( That really have very minimal contact surface area )  Do this enough times and wear ensues causing breech gap to close until you again get the metal on metal of barrel against main tube.

FWB with there 124/127 rifles likely had the most precision in this department using precision ground over hanging ledges of full pivot slot width to index lock up position.   

[hogrider04]

OK guys, thanks for the responses. You've convinced me that mine has the normal Crosman lockup. Now I need to learn to be more gentle when cocking it. I think now I have a better idea on how to  cure my sons Nitro Venom. It has a little gap at the top of the breach and severe barrel droop.  With the information you have given me, I think we can cure both issues. Thanks again.