Tunnel range and change of POI

[NZVarminter]

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?

 

[Mozella]

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.

 

[Ballisticboy]

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.

 

[geraldgee]

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.

 

[Keith Glasscock]

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.

 

[Hal]

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

Hal

 

[243Mendoza]

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?

 

[mr45man]

Mirage . would be my guess.

 

[Ned Ludd]

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.

 

[BoydAllen]

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.

 

[NZVarminter]

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.

 

[Depart123]

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.

It is my contention that what you are seeing is the light push from the sun being eliminated shooting through the tunnel. That is why it is consistent across several different rifles, it is an optical effect rather than a ballistic one. I was just talking to DelFuego about this in another thread (post#40).http://forum.accurateshooter.com/threads/reading-mirage.3935046/page-2 .

If you zero in strong light then your rounds will strike 1-1.25moa high in the tunnel. If you zero in the tunnel, you will see them strike low by the same amount. If there is a rising mirage while shooting out of the tunnel, it will counteract some or all of the sun push and you could see no change at all.

 

[mbz hunter]

I have a feeling that it is a issue with the air friction in the tunnel. This would be like a snorkeler trading in a skinny snorkel for a fatter one, and one as short as they can get and still get above the surface of the water. It takes SO much more work to overcome the friction to breathe in the smaller diameter snorkel. An easy way to figure it out is to put a chrono in front of the target at each location that you shoot. If there is a variance, you might have your answer.

 

[okie]

I have a feeling that it is a issue with the air friction in the tunnel. This would be like a snorkeler trading in a skinny snorkel for a fatter one, and one as short as they can get and still get above the surface of the water. It takes SO much more work to overcome the friction to breathe in the smaller diameter snorkel. An easy way to figure it out is to put a chrono in front of the target at each location that you shoot. If there is a variance, you might have your answer.

 

[okie]

Im just jealous that you have a tunnel!!!
you know the dope, and it is consistent, which is an awesome circumstance.
Would you build me a tunnel that I could test these theories in????

 

[Downhill]

Gonna have to say it is a lighting issue. Best guess.

 

[Cakes]

I do a fair amount of load development in a 100m tunnel as well in the winter months up here in ND. This "tunnel" is underground, 2 sets of approximately 15' wide by 8' high to accommodate 6 lanes. I too have noticed rather odd bullet groupings from time to time. Light/temperature is a constant, I have noticed humidity is quite variable. As each side is 15' across, probably no acoustic wave interference IMO. Some days my Dasher & 284 will shoot in the teens all afternoon and other times it looks like a train wreck. Best I can figure out is there is a very subtle air flow occurring due to ventilation/heating. This has happened with my Dasher and 284, sometimes having a group open up to .75" I hang a small sliver of ribbon on the bottom of the hanger now to look for this....hasn't really helped thou. When the "disturbance in the force" is going on I just pack up and try another time. Eric in DL

 

[243Mendoza]

I do a fair amount of load development in a 100m tunnel as well in the winter months up here in ND. ... Some days my Dasher & 284 will shoot in the teens all afternoon and other times it looks like a train wreck. Best I can figure out is there is a very subtle air flow occurring due to ventilation/heating. This has happened with my Dasher and 284, sometimes having a group open up to .75"...

Has your ammo and rifle reached room temperature before you started? (just making another wild guess at to cause

 

[Cakes]

As to my case, temps are fairly stable. Take the equipment from my truck (mid 60's) and go inside and check in and set up. Takes 10 minutes and also 60's downstairs. It is a "disturbance" of some kind as I've seen a 180 hybrid move .5". My set-up is solid and repeatable and I always shoot in the same lane. One of those "things" I'll probably never understand .
Eric in DL

 

[Delfuego]

Is your tube/tunnel sufficiently ventilated? It would seem that if it wasn't, that you would have considerably more pressure at the end of the tunnel than at your end. By shooting you would be forcing air toward the target end of the tunnel, if there is no where for the air to go vent pressure would go way up. I would also think this would not be consistent with different calibers and that your variations would not be uniform. It may not be uniform, but very minor and therefore appear uniform. If this is actually happening, a larger tube/tunnel and better ventilation, exhaust fan or vent double the size of the end vent, might change the effect. I also could have it totally backward, in that you actually have an velocity advantage with the tube. A 2 foot diameter tube is very small and could act like a suppressor too allowing the bullet to run faster. You can pick up 20fps running a suppressor at my altitude. Pure speculation on my part, but it is an interesting thread and fun to postulate.

I am not a physicist or astrophysicist for sure. I also think I could have it totally backwards because you in the southern hemisphere.​