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Thread: Restricting/tuning the velocity of a QB on HPA

  1. #1
    Senior Member SeanMP's Avatar
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    Restricting/tuning the velocity of a QB on HPA

    This is something I was working on last October and posted on another forum. But as there is a similar thread question I decided to repost it here for reference I was trying to tune the velocity of my AR2078 to match a specific pellet preference. Bob suggested fitting a solid transfer port of 1/4" teflon bar stock, rather than using the standard 1/4" white poly tubing, and drilling a hole in it to a size that would restrict the velocity to my desired number and flatten the shot string. I took Bob's idea and went one step further by milling a 2.5mm deep pocket in the valve body using a 1/4" end mill. I then made a series of restrictors out of 1/4" brass rod with various #drill holes in them.
    restrictor buttons.jpg

    Restrictor Button install in valve body.jpg

    If I had only a drill press press to work with. Bob's idea of using the 1/4" teflon bar stock would be the best and would yield the same result. The only reason I chose to go with the brass ports is because I tear my gun down frequently and I didn't want to have to remake a custom transfer port seal once a week.

    Here are the before and after results at 1500psi fill

    Before restrictorsunrestricted shot string.jpg625fps down to 475fps over 18 shots

    With restrictorsRestricted shot string.jpg512fps to 462 fps over 18 shots AVG 498fps

    I later found that increasing the fill pressure from 1500psi to just under 1600 flattened the string even more and doubled the shot count.

    With the restrictor fitted the performance matches the that of the AR running on C02 caplets. The stock rifle will bounce around either side of 500fps giving a 498fps avg velocity. (which actually impresses the heck out of me that the chinese were able to hold the velocity right at the line)
    Last edited by SeanMP; Mar 13 2012 at 08:12 AM.
    Sean

  2. #2
    Moderator rsterne's Avatar
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    Nice looking transfer ports, I see the O-ring on the side to seal into the valve.... What seals them to the breech?....

    I'm using restricted TPs more all the time for detuning guns.... Set the pressure range with the hammer strike and then tune the velocity with the transfer port diameter....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  3. #3
    Senior Member SeanMP's Avatar
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    Quote Originally Posted by rsterne View Post
    Nice looking transfer ports, I see the O-ring on the side to seal into the valve.... What seals them to the breech?....

    I'm using restricted TPs more all the time for detuning guns.... Set the pressure range with the hammer strike and then tune the velocity with the transfer port diameter....

    Bob
    I'm just using the standard 1/4" white poly tubing for ease of use.

    I love the idea overall. It's a great way to achieve two goals at once. Set the velocity and flattened off the top of the bell curve.
    Sean

  4. #4
    Moderator rsterne's Avatar
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    Restricting the airflow can do amazing things.... The Hatsan AT44, when tuned for Canada, delivers about 180 shots between 455 fps and 470 fps starting from 200 bar and shooting down to 75 bar....

    Get rid of the restriction caused by the thick valve stem, and the same gun, with no other changes, delivers 30 shots between 885 and 945 fps starting from 190 bar and shooting down to 115 bar....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  5. #5
    Senior Member SeanMP's Avatar
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    No kidding!

    Double the speed by reducing the restriction at the valve stem....thats pretty impressive
    Sean

  6. #6
    Member just missed's Avatar
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    Teflon 1/4" bar stock any info on a source, wouldn't mind having some of this on hand in the event I switch my QB78 to HPA. This seems like a very practical means of keeping her under 500fps, well more like something I could do.

  7. #7
    Senior Member SeanMP's Avatar
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    Ya sure I order teflon bar stock from fastenal in 18" lengths for peanuts. Sized from 3/16 up to 1 1/4" I think. It's used in the electrical trade for standoffs
    http://www.fastenal.com/web/products...ex?sku=0955524
    Sean

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    excellent, thanks

    John

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