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Transfer port size ?

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    fishinwrench
    Participant
    Member
    Accuracy: +12

    What effect does enlarging (drilling out) the transfer port have on velocity, recoil, ect. ?

    Seems to me that having the transfer port sized to more closely match the bore diameter would make for a more efficient springer.   Does anyone ever alter that when tuning break barrel rifles ?

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    cherry
    Participant
    Member
    Accuracy: +1

    smaller the hole higher psi ,larger less psi, large hole air is lost at a greater rate I understand what you are thinking i did it lost fps,also air needs to be inside the pellet to seal the outside in the rifling.

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    ranedouglas
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    Accuracy: +13

    …………….   STOP  …………    this area is much more complicated than ever i had dreamed to comprehend …   i completely ruined several irreplaceable english-made webleys experimenting with transfer port dynamics….   there is a lot to learn of this.  i started out by realizing that the turkish webleys i was selling for a while were a bit under-powered due to that transfer port being just a little too constrictive (i understand they were intended for the australian market and should have never been realesed here – per rodger williams who once owned webley when i met him here in north texas) .   using a specially purchased much longer 7/64 drill bit and slowly drilling out those first gen turkish tomahawks , i was able to get around the 1000+ fps with lead that the english-made tomys got…  but spin too fast and the bit gets stuck and you're fudged.  drill too much out and the piston slams – and you're fudged , do anything that just doesnt work out perfectly there – and you're butt-fudged , because changes are for the most-part irrevesible there.  welding the hole back some isnt good – welds are harder than the mild steel tube and dont drill easily…..just on and on and on , man…..  i talk about it a little in the middle of this clip…   if you wanna share with us what gun , caliber , barrel length , and internals you are having this 'idea' for – that would be great – also explain your realistic expectations of performance afterwards..   i cant tell you how many times steve pope has told me , " i believe you might be a bit too ambitious there, paul " in perfect polite brittish english…. .   

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    spysir
    Participant
    Member
    United States
    Accuracy: +78

    Fish,

    You might like to read "The Airgun From Trigger to Target"  . I believe there is now an online copy?

     

    John

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    MDriskill
    Participant
    Member
    United States
    Accuracy: +11

    I will put in another vote for “The Airgun From Trigger to Target,” Cardew’s classic book which has an excellent discussion of this subject. The book used test guns which had the transfer port in a replaceable threaded grommet, an ingenious idea allowing the port size to be easily changed.

    I would also mention that in my observation the subject of transfer port size seems to get more attention from shooters and tuners in the UK, than it does here. The magazines “Airgunner” and “Airgun World” frequently have tuning articles that include some very sage advice, backed up by testing data, on this subject.

    Altering the port size CAN make for improvement, but as ranedouglas noted, it’s a very complex subject! Desired power level and firing behavior, spring strength, and piston weight are all part of a quite complex equation. A bigger port is NOT usually the answer.

    You can get a look at those mags here:

    https://www.airgunshooting.co.uk/magazines

     

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    Scrufhunter
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    Member
    Accuracy: +8

    cherry

    smaller the hole higher psi ,larger less psi, large hole air is lost at a greater rate I understand what you are thinking i did it lost fps,also air needs to be inside the pellet to seal the outside in the rifling.

    The smaller the hole the greater the PSI.  Increasing the force behind the piston works but you can only go so far until do you hit a point of dimessingminishing return . At this point if you.want to increase velocity increasing the piston bore and or stroke length is where the work should be done. This is evident in the newer generation of gas ram tech.  Back in the day I read thread on the 13xxx platform in relationship to the volume in the valve vs that of the barrel, and the optimal length varied between calipers at the same PSI. I don't think anyone has experimented with the volume vs PSI and barrel volume/length and how it relates in performance at least I have not read anything or seen anything any where regarding springers

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    fishinwrench
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    Accuracy: +12

    I am not going to do it to any of my guns but I did have the thought when I last overhauled my Benji Prowler.   There was a burr in the transfer port and I just used the correct size drill bit to remove the burr and make the hole nice and round again.   

    I do know that oversizing and sealing the transfer port on my Crossman 1322 really "woke it up".  So I was just curious if anybody tinkered in that area much.

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    ranedouglas
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    Accuracy: +13

    …..  i had wondered that having a hex-type screwin for transfer ports would be really cool….with different size air channels….especially if you had interchangeable caliber barrels and perhaps different power-leveled gas rams or even springs…  a chocked barrel makes a difference , too…..   but yeah , i bought a used 2240 type that customized…..man , i couldnt believe how big that transfer port was ! idk how the pellet didnt fall into that while the bolt pushed it thru….later i spoke with a fella that even made small wire bridges for his pellets to scoot over that crater…..  but with co2 , pcp , and such – it's a much eaier science………   just imagine what all those whiscombe air rifles entail….i think he even harmonic balances the barrels…..    i'd need a piece of lead that is like a thick piece of leather to temporarily place here and there to find out how to crudely balance some of these 'piston' guns…   

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    Scrufhunter
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    Accuracy: +8

    I have wanted to modify a zerk fitting or some kind of nipple and and drill n tap the piston side and have it protrude into the barrel and rest behind the pellet. I suppose it would seat the pellet and by pass the breach seal. Put more PSI behind the pellet right into the skirt. Maybe one day I'll try it.  I think I may have seen something like that on an old under lever a buddy had sitting on his porch awhile back

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    Scrufhunter
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    Accuracy: +8

    Anyways you can only go so far in velocity really should stay below super sonic unless it's possible to push the pellet 3-4 Times the speed of sound, but I don't think that will be possible with a spring or gas ram…I think the future of airguns will be an advancement in ammo. More bullet boat tailed shape I think will be the progression, at the moment most pellets who share the diabolo shape are not the most efficient for super sonic flight. 

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    TiredRooster
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    United States
    Accuracy: +10

    This is a very interesting topic. I would like to read that book, "The Airgun From Trigger To Target" myself. The British air gunning community is very large due to the strict firearm controls there. In addition to that, the restrictions placed on air rifle energy has led to a ton of innovation and quality air gun smithing. Thank goodness we are not subject to that sort of thing here. However, there is a lot that can be learned from the Brits in the area of "springer" tuning and improvements.

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    MDriskill
    Participant
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    United States
    Accuracy: +11

    Regarding shortening/re-positioning the transfer port, two interesting designs you might enjoy looking at are the famous FWB 300S match rifle, and the little-known Haenel 312 target rifle made in the former East Germany.

    On the FWB, the breech is a cone shape which “reaches into” the breech seal, so to speak. This has the effect of shortening the transfer port’s length to almost zero; the head of the piston comes within 1/8” of the pellet when the gun is fired. This is a key ingredient to the recoilless action of these guns, as it greatly reduces the necessary spring strength, and thus how much recoil must be overcome. These guns shoot well over 600 FPS and you can literally cock one with your little finger, which is quite amazing.

    The Haenel has frankly the weirdest transfer port and breech seal design I have ever seen. It has a fixed barrel with sidelever cocking and sliding breech sleeve similar to the FWB, but the transfer port is a long “finger” which also serves to push the pellet into the barrel as you describe. The gun has no resilient breech seal material, but reduces leakage at the breech by creating a such a long and angular path for the air to escape there, if that makes sense. 

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    h2rider
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    Accuracy: +0

    Mike,

    I think that type of seal is referred to as a "convoluted" seal, or some other similar term.  A few multi-cylinder two stroke engines use the same type of seal between adjacent cylinders.  They are not perfectly air-tight, but for the short duration of the firing cycle, the leakage is minimal.

    Paul

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