Learned something today!

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  • #2140
    mm123522
    Participant

    I am building a 9 mm talon style rifle. The breach was moving very hard, because the front “o” ring was tight. At this time I was getting 850 FPS with 77 grain pellets.
    I decided to “loosen up” the front breach “O” ring by cutting the “O” ring groove a little deeper. That made the breach move much better, but when I shot it again the velocity droped off drasticaly.
    Air leakage was getting between the breach and the hammer and was killing the velocity. I cut another “O” ring groove and installed another “loose” “O” ring. The velocity was still bad, about 550 FPS.
    Finally I cut vent grooves in the face of the breach to prevent pressure from developing between the hammer and breach face. The velocity went right back up to 850 FPS!!!
    The conclusion is: Leakage of air from the front , breach “O” ring will push the hammer forward prematurely closing the valve, and limiting power.

    Check out the pictures – see the 4 vent grooves on the breach face.
    Tested the gun at 550 FPS
    Cut those 4 grooves, and did nothing else.
    Tested again at 850 FPS.

    Mark

    #39061
    shrpshotr28
    Participant

    That double o-ring setup should be a keeper with a little tweaking to get the grove/oring sizes right I imagine you won’t need the vents in the next one. That is actually a really good idea considering the air volume to move that big pellet. (yes I understand it wasn’t intended and is two separate attempts at sealing the breach but maybe could be incorporated on purpose in the future).

    I need more tools 🙁 I want to be able to experiment like that.

    #39068
    voltar-1
    Participant

    Mark, I’m a little dense on this idea. Could you explain it another way perhaps?
    Do not understand what your theory is.
    Walter…

    #39073
    shrpshotr28
    Participant
    quote Voltar_1:

    Mark, I’m a little dense on this idea. Could you explain it another way perhaps?
    Do not understand what your theory is.
    Walter…

    I’m not Mark, but understand the theory. His front breach o-ring is not sealing adequately, and the air pressure making it past the o-ring is pushing FORWARD against the hammer/spring; reducing REARWARD pressure to hold open the valve, thus reducing valve duration and decreasing velocity.

    #39076
    voltar-1
    Participant
    quote shrpshotr28:

    quote Voltar_1:

    Mark, I’m a little dense on this idea. Could you explain it another way perhaps?
    Do not understand what your theory is.
    Walter…

    I’m not Mark, but understand the theory. His front breach o-ring is not sealing adequately, and the air pressure making it past the o-ring is pushing FORWARD against the hammer/spring; reducing REARWARD pressure to hold open the valve, thus reducing valve duration and decreasing velocity.

    Well if that were the case the force created by such is pushing reaward on the tophat holding the valve open.
    Still befused, thanks for trying eh! 🙂

    #39082
    dogwood
    Participant

    more than likely they are allowing trapped air to escape and allowing the o-ring to do there job and seal

    or I should say allowing the air to escape quickly enough to allow the o-ring to deform and seal

    #39088
    knifemaker
    Participant

    Looks like it would vent air in your face.

    #39091
    blackwidowman
    Participant

    I’m with Walter on this one (not because I’m canadian too eh!)
    But the hammer has no seal and if air did escape by the front it would go around and through the hammer using the less resistance path, unless you had a large volume of air escaping.

    Yves

    #39105
    mm123522
    Participant

    Guys:
    I dont know what is happening ether!??
    I know I was getting around 850 FPS, but I had a tight front breach “O” ring. this caused the breach to move hard.
    I cut the front “O” ring groove a little deeper and the breach moved better, but the velocity dropped a lot.
    I put in another “O” ring figuring that I was loosing air – still had poor velocity.
    Sense the only change, before loosing velocity, was cutting the “O” ring groove, I still figured I had an air leak problem. So I cut the vent groves and the velocity went right back to 850 FPS.
    I dont know what the mechanism of the velocity loss was, but the vent grooves appeared to fix the problem????????
    Any ideas????

    My hammer rides on the barrel, not the I.D. of the tube. There is about .020″ clearance between the barrel and the hammer.

    The hammer face is very flat and mates with the flat breach face.

    I dont notice a big blast of air from the breach when I fire.

    Tonite I will cut the vent grooves deeper and see if it makes any difference.

    Mark

    #39110
    walkonking
    Participant

    What the Hell?

    I am lost and would love to have a clue what is being discussed 😆

    #39127
    rhoderman
    Participant

    You suppose all them chips and chit are interfering with making a good seal?
    I’d be cleaning it all up. Also, one of your pics looks like the o-ring is notched. It might be the light though.

    I don’t understand how notches where you put them would accomplish anything.

    A tight o-ring might allow it to hold the valve open longer simply due to friction.

    How is the seal at the tophat end? Seems like that could be another part of the equation… Another thing: if your cocking knob is binding, it will cause a big decrease in velocity.

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