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Ballistic coeficient question with data

 
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  #1  
Old 12-16-2005, 06:35 PM
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Ballistic coeficient question with data

Well, boys, out there in .com land I need, or at least want, some ballistic coefficient info. First the rambling.

Today I fire formed two boxes of 7MM STW using 130 grain G.S. Custom H.V. bullets. Two boxes of ammo were taken from the safe. One box was wrapped in a couple of towels and put into an insulated lunch box. The other, with the lid off, was laid on the windshield between the wipers. Im sure by the time I went fifteen miles to the bank, stopped to mail some T shirts and drove back a dozen miles or so to the range the ammo was down to 27 degrees like everything else outside. The 80 grains of IMR7828 were sparked by Federal 215 Magnum primers in Midway bulk cases. The home made muzzle brake with seven rows of seven holes (.7x7mm diameter) drilled at 2x7 degrees angled back WORKS! The sky screens for the Ohler 33 were centered twelve feet from the muzzle. I was delighted to see the bullet holes at 300 yards dispite the light fog. The 3-15X Sightron II constantly gives me buyers congradualtions. The range is about 1,000 elevation. There was heavy overcast with fog limiting visibility to about half a mile.

The room temperature loads averaged 3,568 feet per second. The cold ones surprised me by achieving 3,516 feet per second. It appears one looses about fifty feet per second when the temp drops 45 degrees. Someone wanted to know what the velocity would be at 200 yards so I shot a three shot group at that range then placed the sky screens under the group with the center six feet from the target; or 198 yards from the muzzle. Of the eight cold shots fired at 200 yards, seven gave a reading which averaged 2,958 feet per second. Maybe some of you computer whizzes could calculate the ballistic coefficient from this data. But there is more.

Both 100 yard groups were 2 5/16 above center. The seven shot 200 yard group was 3 3/8 above and the 300 yard four shot group was 9/16 above center. The first shot at 200 yards was about 2 higher than the group. The first shot at 300 yards was 3 higher than the group. The 200 yard group was fired after about twenty minutes of cooling and the 300 yard group after another half hour. Neither were included in the measurements. The four I did measure at 300 made a 1 3/16 group.

I certainly hope there is enough data to calculate the B.C.
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Old 12-16-2005, 08:39 PM
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Re: Ballistic coeficient question with data

To make sure I have it all right: 1000 ft alt, 27 degrees, 4 yd velocity 3516, 198 yd velocity 2958.

Equates to about a .365 sea-level corrected (ICAO) BC.
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Old 12-16-2005, 09:29 PM
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Re: Ballistic coeficient question with data

Sometimes it can be hard to determine an accurate BC using 100 yard velocity and 200 yard velocity. Often times the bullet hasent stabilized and settled down yet. We all know that a bullet with even a slight wobble will not have a good BC. Once the bullet settles down, this is where you are going to find its usable BC.

The best way to determine a BC (IMHO) is to fire your loads at a variety of distances all the way to your max range and adjust the BC in a ballistic calculator after entering all you other known variables till you find what matches your real world results. Its best to use a program that has multiple drag functions.
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Old 12-17-2005, 12:29 AM
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Re: Ballistic coeficient question with data

Certainly true, but I think an accurate, measured average BC over the first 200 yds will give better data than 99% of us are using. 50, even 100, probably not as the bullet could still straighten out significantly, but 200 is pretty good IMHO. Those first 200 yds are where drag forces are the highest and the BC during this period has cumulative effects (even at 1000+ the flight of the bullet has still been significantly affected by its BC during the first 200 yds).

In any case, virtually all of Sierra's published BC's are average BC's from measuring over the first 150 yds. It's just pretty impractical to measure them at really long ranges, even with an underground range.
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Old 12-17-2005, 09:42 AM
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Re: Ballistic coeficient question with data

What was the relative humidity and the barometric pressure? [img]/ubbthreads/images/graemlins/confused.gif[/img]
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Old 12-17-2005, 10:01 AM
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Re: Ballistic coeficient question with data

sniper2,

I hope this helps. It came off the web for my area when I came home.

Altitude: 1000 ft

UV Index: 0 Low
Wind: From NE at 2 mph
Humidity: 77%
Pressure: 29.93 in.
Dew Point: 32F
Temp: 27
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Old 12-17-2005, 01:58 PM
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Re: Ballistic coeficient question with data

I too roughly came up with .365 based on the G1 curve. Seems a little low. But thats what the math says!
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Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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