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PostPosted: Fri Feb 06, 2015 9:42 am 
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Joined: Mon Dec 15, 2014 10:20 am
Posts: 24
Ok so I. Trying to under stand the BC rating system do u want a higher or a lower number for the BC? I'm pretty sure u want as close to 1.0 as possible right any help here plz?


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PostPosted: Fri Feb 06, 2015 10:44 am 
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Joined: Sun May 04, 2014 1:56 pm
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Location: British Columbia
Google It. Everything you ever wanted to know about BC is just a few key board strokes away.


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PostPosted: Fri Feb 06, 2015 11:05 am 
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Location: Coalmont BC
The BC is a comparision of how your pellet stacks up against a "standard projectile", which has a BC = 1.000.... The higher the BC is, the more aerodynamic the pellet/bullet so the less drag at a given velocity.... This leads to less wind drift, and less loss of velocity and energy as the pellet travels downrange, and a flatter trajectory for a given muzzle velocity.... The drag and drift are not constant when you change the velocity, however....

If you measure velocities at the muzzle and downrange and use a calculator such as the one in ChairGun to calculate the BC, you will find that it is not a constant, but varies with velocity.... This is because the drag coefficient is not a constant (although it is relatively so between 500-800 fps) but increases dramatically as you approach the speed of sound (for both bullets and pellets).... When you find different BCs for different velocities what you are really discovering is that your pellet/bullet doesn't match the drag profile of the standard projectile very well.... Chairgun has recently introduced a new drag profile called "GA" which is supposed to match pellets better than the traditionally used "G1" profile for this reason....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
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Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
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PostPosted: Tue Feb 17, 2015 9:32 am 
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Joined: Mon Dec 15, 2014 10:20 am
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Sweet thanks a lot!


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PostPosted: Wed Mar 04, 2015 6:27 am 
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Joined: Sat Jan 25, 2014 9:16 pm
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Location: United States
Some info that might help:
BC for some common pellets:
http://www.photosbykev.com/wordpress/us ... etdata.htm
And the attached pix for low powered .22.
Sometimes they don't make sense, and the best way to do it is with your gun and two crony's to measure muzzle and whatever down range velocity. The better the BC the less loss in speed you'll see. That Chairgun program is really cool, but you kinda need the BC and your guns actual velocity, so basically you need at least one crony. Or just guess your velocity and use BC from one of the charts.
Then there's the drag chart at the bottom which shows how things change with speed, so in this chart ~900-950fps the bullet has the least drag, then drag skyrockets at it nears and goes thru mach. That chart was for a much heavier bullet but it's typical. So the link at the top shows BC but no velocity so we don't have enough info. The chart below gives velocity so if your velocity is close I'd use that.


Attachments:
Ballistics Coefficient low power 22.jpg
Ballistics Coefficient low power 22.jpg [ 115.42 KiB | Viewed 804 times ]
Drag chart 2.jpg
Drag chart 2.jpg [ 103.37 KiB | Viewed 804 times ]
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PostPosted: Wed Mar 04, 2015 11:09 am 
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The best velocity for pellets is much lower than for that bullet, by the time a pellet gets to 950 the drag is already going up rapidly.... Otherwise the advice is sound....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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