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Old 10-03-2022, 12:23 PM   #1
Notaguide
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Default POI shift indoors vs. outdoors

Over the last month I visited a new indoor range that opened up recently working up several loads. After shooting a couple of rifles having zero'd indoors, I experienced a 3" shift outdoors. The two guns weren't exactly consistent in the shift but are both low while outside. The first time I experienced it I shrugged it off. Now I question the lighting or other variables after a second visit. Curious if anyone has shot 100yds indoors and experienced this?
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Old 10-03-2022, 02:56 PM   #2
muzzlebrake
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Could have something to do with the ventilation indoors. Most have some sort of exhaust system that removes and cleans lead and powder burn fumes and particulates from the air. I have designed a couple of indoor range exhaust systems used in DPS forensics buildings back before I retired. Air flow velocity is generally around 400-600 fpm to get full evacuation of the cross sectional area. The bigger the space/volume, the more air. Usually a higher velocity than you would see on commercial kitchen vent hoods. Most indoor ranges should have a fairly high negative indoor air pressure so when you open the door you would feel a swoosh of air coming in.

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Old 10-04-2022, 04:48 PM   #3
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at first based on muzzlebreak's response i'm thinking barometric pressure .. but 3" at100 seems like allot...
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Old 10-06-2022, 09:40 AM   #4
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Wind?
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Old 10-06-2022, 12:02 PM   #5
muzzlebrake
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Quote:
Originally Posted by chrisk View Post
Wind?
600 feet per minute exhaust air velocity is 6.8 miles per hour. Certainly enough to cause windage shift up down or sideways. Would also depend on location of exhaust inlet and makeup air inlet.
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Old 10-06-2022, 12:11 PM   #6
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A properly designed range exhaust would have some sort of laminar flow that extends end to end of the range. A 100 yd indoor range exhaust will be quite expensive and unless it's tax dollars paying for it or local code dictating it then it's probably a cheaper less effective design. Air in at one end and all sucked out at the other end. Something like this is more than most will use or pay for.
https://youtu.be/s3b3KlV9UZg?list=TL...5OQwNjEwMjAyMg
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Old 10-06-2022, 03:16 PM   #7
Notaguide
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The comment was made by employees that the exhaust system was one of the largest costs during design and construction.

I was leaning towards a light or shadow issue until the ventilation idea was mentioned. If the issue had not occured on multiple rifles I would never thought much about it. It really came to light after shooting a match .22lr at 50yds t the range a found it 2" low later after my son missed a shot.

There was only one discussion I could find when searching the interweb about shooting in a small tunnel with similar results.
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Old 10-06-2022, 04:10 PM   #8
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Quote:
Originally Posted by muzzlebrake View Post
600 feet per minute exhaust air velocity is 6.8 miles per hour. Certainly enough to cause windage shift up down or sideways. Would also depend on location of exhaust inlet and makeup air inlet.
6.8mph equates to 10ft/sec ... don't think 10ft/sec will get you 3" @100 yards
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Old 10-06-2022, 04:28 PM   #9
Mike D
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I would think several environmental factors could be at play here.

Temp
Humidity
Barometric Pressure
Wind (or lack thereof) influences.


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Old 10-06-2022, 04:35 PM   #10
bowaddict40
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Ive often thought the harmonics coming out of the rifle made a difference on indoor ranges. I have never zeroed a rifle indoors and had it be the same outdoors. That tube at BPS is the worst.
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Old 10-07-2022, 08:59 AM   #11
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Quote:
Originally Posted by imyomama View Post
6.8mph equates to 10ft/sec ... don't think 10ft/sec will get you 3" @100 yards
Would depend on the size and location of the makeup air. Generally the makeup is toward the end of the range and exhaust at the sides or near the shooting bench in order to capture smoke and particulates. Makeup air in the ductwork is much higher velocity than the air moving thru the cross sectional area of the space or room tunnel. air velocity inside ductwork is usually around 1000 to 1500 fpm to keep ductwork from being so big. A big makeup air unit sitting on the roof right above the target area and down blasted into the space could make for some serious air currents. This is all assuming the range went el cheapo in with some contractor design and since proper range ventilation systems are enormously expensive. Now if the range is both heated and cooled it gets even more complicated. Conforming to government regs and ASHRAE and UBC is for the big boys with lots of cash. https://assets.publishing.service.go...ap30_DLRSC.pdf
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