Remember I posted a suspicion that the AA piston may smoke scope objectives, well...

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Thread: Remember I posted a suspicion that the AA piston may smoke scope objectives, well...

  1. #1
    Lead Moderator Array Tangle's Avatar
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    Remember I posted a suspicion that the AA piston may smoke scope objectives, well...

    First a refresh. I was at the range one very misty day at the 200 yard range shooting my 11.5" Voodoo bbl with the Adams Arms piston kit on it. BTW, I have yet to have a malfunction of any kind with this kit, and I can't remember the last time I cleaned it or lubed it and it still adjusts freely. Anyway...

    I was having a hard time seeing the targets. I could see the reticle clearly and sharply so I figured it was the effect of mist over 200 yards. Then I looked an my objective lens and it had something on it??? It was a film, had some oil in it and I liked to have never got it cleaned off. Of course I was trying to be verrrrry careful not to scratch the lens or damage the coatings.

    That's what made me think the piston exhaust might be smoking the lens. Adams Arms said they had heard of that. And let me make this clear, this has only happened on a longer scope with the objective very close to the piston. Here's what the block and piston look like:



    So to pursue this issue, I decided to combine two things. My boss is big on 'cross pollination", i.e. acquiring knowledge outside our discipline and one thing he especially wanted is some familiarity with our 3D printers. So, hmmm, what if I print out a guard to slide over the rail and shield the piston! So I crank up SolidWorks and learn to use it for the design, and before I know it, I have the first rail prototype. From here it evolved into this:



    Even the hole for the 1/8" roll pin is printed into the part. I printed the shield in clear, or as clear as it gets.

    Here's an end view:



    So, it's off to the range to test my theory and use the poly shield. I shot a bunch with the poly shield in place and had some concern the heat from the gas and the heat build up in the gas port would melt it but it didn't. Although I could start to smell 'plastic'.

    Well, here's what I found:



    You can clearly see the plastic is 'smoked'. That doesn't mean the discharge is fogging the objective, but it does prove there is some exhaust coming from the piston. And, I've never had an objective smoked before.
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  2. #2
    JD
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    Good stuff!

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    VIP Member Array Smitty901's Avatar
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    Never had the issue with My AA ones but will keep this in mind.

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    VIP Member Array Hoganbeg's Avatar
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    Of course. The gas has to go somewhere. If not onto your BCG, then out the front.
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    Thanks for sharing Tangle, interesting piece you made.
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    That is really smart work.

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    Good job! You recognized a problem and demonstrated a solution - probably worth filing for a patent on the idea. Maybe we'll see it in the next Midway or Brownells catalog!

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    Lead Moderator Array Tangle's Avatar
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    Quote Originally Posted by HotBrass45 View Post
    Good job! You recognized a problem and demonstrated a solution - probably worth filing for a patent on the idea. Maybe we'll see it in the next Midway or Brownells catalog!
    Hmmm, I'd better give that some thought!

    I also machined down a short riser and used that as a guard and it works well too, but I like the compactness and low profile of the printed guard.
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    Lead Moderator Array Tangle's Avatar
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    Quote Originally Posted by Hoganbeg View Post
    Of course. The gas has to go somewhere. If not onto your BCG, then out the front.
    Exactly, but up to this point, I have seen no mention of gas exhausting from the rear of the gas block. In fact, IIRC, AA says something on the order of: the gas vents away from the shooter. I assumed this meant the gas vents from the front of the gas block and toward the muzzle. It's also possible that after the bullet leaves the barrel, some of the gas could exhaust back through the barrel.

    Anyway, the exhausting gas from the rear of the gas block tends to 'deflect' rearward, unfortunately toward the scope's objective.

    Now, I do not shoot a piston operated AR without a guard on it.
    Aceoky likes this.
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