TargetTalk

a place to talk about Olympic style shooting, rifle or pistol, 10 meters to 50 meters, and whatever is in between.
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PostPosted: Thu Aug 27, 2009 9:03 am 
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Location: Virginia
Several of us at my club are talking about initiating procedures for outdoor air rifle shooting at 10 meters. For example the new NMAR program. Some people said "the wind will blow your pellets off the target", so I ran one of the online calculators to see some facts. It gave a result of 0.138 MOA (0.40mm) windage at 10 meters with a 10 mph wind, 50 feet above sea level, 72F, 80%humidity, 8 grain pellet. I used a ballistic coefficient of 0.5 (pure guess) and a modest muzzle velocity of 501 fps.

Can anyone out there do a better job of running this calculation for me? Does anyone know the ballistic coefficient of a .177 pellet?

The number 0.4mm sounds right to me. The pellet size is so small and the 10m range is so short (flight time 0.66 sec) that the wind has very little time to cause drift.

If you actually know what you are talking about please provide a better answer.


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 Post subject: Wind drift
PostPosted: Thu Aug 27, 2009 1:07 pm 
The ammount of wind drift is not based on flight time so you should discard that omputation. The thery [sic] is based on the amount the projectile slows on the way to the target. Start with muzzel velocity and the remaining velocity at the target. Subtract target velocity from muzzel velocity and you have the time the wind can affect the pellet. The concept is bassed on the fact that in a vacum there is no lose of velocity so the velocity changes are based on atmosfere. This includes wind velocity , direction ,humidity, how far one is above sea level, and temperature. good luck with your project. Good Shooting Bill Horton


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PostPosted: Thu Aug 27, 2009 3:55 pm 
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We shoot 10m rifles outside for Target Class Silhouette all the time out to 45 yards for the Rams.

1st off your ballistic coefficient is way off. For a JSB lite Dome headed pellet of 8.4g your looking at roughly a .022 BC and with a velocity of 600 fps which is what my Steyr spits them out at and the time in flight is around .057 seconds for 10m. With a 10 mph cross wind from 90 degrees your looking at a drift of around .309" for 10 meters and .154 for a 5 mph wind.

Even at 45 yards the flight time to target is only .25 seconds. But let me tell you the wind can really play hell on those lite pellets moving so slow at that range with drifts of several inches possible in breezy conditions, not all that often it is calm enough to hold on the target center itself but the nice thing is with the slow velocity and high mag scopes we can spot for ourselves, at time you have to aim an ich or more off the Ram. I have had pellets start drifting right then up then back to the left before getting out to the 45 yard mark on windy days with it swirling, makes for some very challenging wind reading to say the least.

So yes it is possible of course to shoot 10m outside and the drift will be evident even more so at 500 fps but I hope this gives you a better idea of what the reality is using these rifles outside. Using a rifle designed for shooting 33' inside to shoot 135' outside makes it very fun and quite the challenge.

Google "Chairgun" for a nice shareware app that gives you all sorts of data for pellet gun shooting ballisitcs if you want to see all the variables and enter some of your own.

Bo


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PostPosted: Fri Aug 28, 2009 4:33 am 
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Bo, you gave me what I needed, thank you. With a BC of 0.022 I now calculate 7.5mm drift. We just saw a video of Silhouette shooting and there is a lot of interest being generated. I think we will start with paper first and then try to move into silhouette slowly.

Thanks again for the info.


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PostPosted: Fri Aug 28, 2009 5:11 am 
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Location: Herts, England, UK
Old Dog wrote:
Bo, you gave me what I needed, thank you. With a BC of 0.022 I now calculate 7.5mm drift. We just saw a video of Silhouette shooting and there is a lot of interest being generated. I think we will start with paper first and then try to move into silhouette slowly.

Thanks again for the info.


Umm, that sounds like a lot of drift to me at 10M. We have odd competitions here where we shoot 10M AR & AP outdoors and I've never noticed such a large movement, and 10MPH isn't especially windy.

Rob.


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PostPosted: Fri Aug 28, 2009 7:50 am 
Look what 10mph wind will do too a .22lr at 50m and say it's not much ??


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PostPosted: Fri Aug 28, 2009 10:02 am 
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Anschutz wrote:
Look what 10mph wind will do too a .22lr at 50m and say it's not much ??


I don't know how much, but we're talking 1/5 the distance here.

Rob.


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 Post subject: Wind drift concept
PostPosted: Tue Sep 01, 2009 9:59 pm 
Please read and consider my earlier post. You are wasting your time trying to relate the ammount of drift based on time of flight. Any long range shooter can confirm the uselessness of time of flight as the prime relsationship for bullet deflection. I repeat that it is the difference between muzzle velocity and the loss of velocity when the projectile arrives at the target that is the time the atmosfere has to affect the drift of the projectile . Good Shooting Bill Horton


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PostPosted: Tue Sep 08, 2009 4:14 pm 
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Location: South, UK
10mph crosswind... 10m, at 600 fps... i'd be thinking of aiming off a tad... but being an FT shooter it's not something I measure, more I feel.

at 800 fps a 10mph crosswind takes 6" off a 55yd target.

look up chairgun and download, this will do the work for you.


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