Tunnel range and change of POI

Discussion in 'ELR, Ballistics & Bullets Board' started by NZVarminter, Sep 29, 2017.

  1. NZVarminter

    NZVarminter

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    I've built a 100m tunnel range with 2 foot diameter pipe.

    Works great to catch the noise and no wind so reducing one more variable for load development. But I've noticed my zero point is different on my range compared to my local club range.

    IF I zero sop on at my range at 100m, my POI is 3/4" low out at the club range.

    This different is consistent with 3 different rifles (22BR, 6.5 SLR and 284)

    Initially I thought it might be barometric conditions, but there isnt enough diff (997-953 mHg) to make that amount of different. Then I got to thinking that maybe the enclosed space is affecting the bullets flight path?

    Could that be a factor?
     
  2. Mozella

    Mozella

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    Short answer: I don't know for sure.
    However, since this is the Internet, I'll take a guess. You can hear subsonic things, like trains, coming at you because as they disturb the air molecules between you and them, that disturbance is transmitted from one molecule to another at the speed of sound. In other words, a sound wave is normally pushed out ahead of the train.

    But if the object is supersonic, like most of the bullets we shoot, the interaction of the bullet and the air molecules (sound wave) can't travel out ahead of the bullet since the molecules "communicate" with one another at the speed of sound. In fact, that's the essence of the speed of sound; i.e. how quickly they "communicate" with one another. Since the bullet you shoot is traveling faster than the speed of sound, the air ahead of the bullet, in other words the air inside your pipe, can't "know" that the bullet is coming. The air ahead of the bullet is just sitting there fat, dumb, and happy; therefore, the air inside the tube ahead of the bullet should have no effect on the flight path...................... I think.

    It will be interesting to see the opinions of others.

    Arguments that you might have air flowing through the pipe will not be persuasive. A wind inside the pipe strong enough to cause a 3/4" shift in POI would blow your hat off.
     
  3. Ballisticboy

    Ballisticboy

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    Since your POI is always low it sounds like the bullets in the tube are being affected by the top of the tube. You may be getting tunnel wall/bullet interference of some kind. Have you tried changing the target position or the rifle position to see if it changes your POI when firing down the tunnel?
    Assuming your bullets are well supersonic I would not expect shock interaction from bouncing shocks off the tunnel roof to be causing the problem or any shock compression of the air ahead of the bullets. Is the temperature inside the tunnel the same as when you are shooting outside?
    The difference is, I would guess, most likely due to a combination of factors.
     
    Last edited: Sep 30, 2017
  4. geraldgee

    geraldgee Silver $$ Contributor

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    Allow me to hazard a guess: You don't mention if the target is outside the tube - in other words shooting through the shadows of your pipe to a naturally lit target at the tube's end. I don't know the science of optics but I suspect it might have something to do with how your eye (looking through the scope) perceives your intended POA through the tube vs. the range at your local club. OR - it may be something entirely different.
     
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  5. Keith Glasscock

    Keith Glasscock True believer - Straight 284 Gold $$ Contributor

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    How close is the bullet passing to the bottom of the pipe? My guess is that this could be similar to how a Magnetospeed bayonet affects poi.
     
  6. Hal

    Hal

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    The way the light shines on your target can change the point of impact.

    Hal
     
  7. steve_podleski

    steve_podleski

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    is the tunnel above the ground or below the ground? If above, the sun may heat the air inside the tunnel and cause mirage...do you see any mirage?
     
  8. mr45man

    mr45man

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    Mirage . would be my guess.
     
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  9. gstaylorg

    gstaylorg Silver $$ Contributor

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    This may seem like a stupid question, but are you absolutely certain both distances are exactly 100 m? I have shot on plenty of local ranges where the 100 yd range wasn't even close to accurate in either yards or meters. I guess close enough was close enough when they were built ;).

    Otherwise, I'd guess Keith might have the right idea as far as some effect of muzzle blast inside your tunnel, similar to the way a MagnetoSpeed chronograph can influence POI. If you have a rifle with a muzzle brake that vents either to the side or slightly backwards, you might see whether that rifle also shows the same behavior. People using brakes of those types have noticed a much lower effect on POI when their MagnetoSpeed is attached. By venting some of the exhaust gas to the side, a muzzle brake can attenuate the muzzle blast in a forward direction that changes POI, so I'd expect the effect to be noticeably less with a brake if it is actually caused by the muzzle blast inside your tube.
     
  10. BoydAllen

    BoydAllen Gold $$ Contributor

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    I would be interested in the temperature of the wall of the bottom of the tunnel, and the air in the tunnel. When you have a difference in temperature between the tunnel wall and the tunnel air, you can get an optical effect that is likely to displace the targets image. This would correspond to what mirage shooters describe as a boil. Lower magnifications and shorter distances as well as different illumination may effect the visibility of mirage. A friend has a professionally engineered shooting tunnel. There is an expansion room between the shooting room and the entrance of the tunnel, and he told me that the engineers told him that a smooth tunnel wall was the preferred configuration. I am not sure why, but perhaps corrugation would increase the effect of a difference in temperature between tunnel wall and air . Another factor that he discovered was how the tunnel was vented. Air was pulled from the tunnel at the target end, and refining that flow had an effect on groups. My friend's tunnel is in a desert region of southern California and is entirely underground and constructed of concrete. It is air conditioned and the AC must be run for several days (I forget exactly how long) before the tunnel comes to a stable condition that is suitable for the highest level of testing. The monthly cost of AC is more than most would afford. Tunnels are a lot more complicated than most would think.
     
  11. NZVarminter

    NZVarminter

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    Well I did some more testing today. Shot in my tunnel range and also directly alongside, and yep, exactly 1" higher in the tunnel. No mirage at this time of year, but I definitely get distorting in middle of day in summer in the top if the arch, so the air does heat up. I left the rifle in the rest sighted on the target and its not moving off so dont thing mirage is the issue.

    tested with 22 BR and 6.5 SLR again today and had 3 shot groups go 0.3" so doubt its variable mirage.

    I can only guess that its related to the confines of the tunnel and the shock wave, but hard to imaging there is any influence in front of the bullet. I have a 6 foot long blast chamber with 6" square hole to shoot through, so maybe the blast chamber is influencing the the initial path of the bullet.
     

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