 ballistic coefficients? 

08242003, 08:29 AM

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Join Date: Aug 2003
Location: Devine, TX, USA
Posts: 11


Re: ballistic coefficients?
Thanks Darryl, appreciate it.
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Steve Moody

08242003, 08:48 AM

Platinum Member


Join Date: May 2001
Location: Pennsylvania
Posts: 1,757


Re: ballistic coefficients?
Steve
I have found that, with any listed BC, it's best to "actual fire" to a WELL known yardage and check the drops for "your" rifle as you have been doing.
You can then go back to the ballistic program and find the BC that the bullet is producing in your rifle and during the weather conditions and tempertures you fired in.
Some BCs do better and some do worse then what is listed.
Fire for effect.
Later
DC [img]images/icons/smile.gif[/img]
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Darryl Cassel

08242003, 10:19 AM

Silver Member


Join Date: Aug 2003
Location: McKinney TX
Posts: 408


Re: ballistic coefficients?
Here’s the real issue when it comes to BC’s. Bullet manufactures use a G1 drag coefficient. The G1 drag coefficient and the mathematical calculations were developed for flat nose projectiles such as the 45 ACP bullets. The G1 Drag yields high BC numbers.
Boat tail bullets use the G5 drag model. The G5 drag model and the mathematical calculations were developed specifically for boat tail bullets, but they produce a lower BC numbers because of the mathematical calculations involved. VLD (Very Low drag) bullets use the G7 drag model and flat base spire point bullets use the G6 drag model.
Bullet manufactures advertise the G1 drag model because of the higher BC number. Higher BC’s sell more bullets. The Sierra .308 168 gr, match king has an advertised G1 BC of .462. The actual G5 drag BC is .289. This is the drag model the US Army uses at the Aberdeen Proving grounds for its calculations. Why, because it is very accurate.
Most of the ballistic programs on the market use the G1 drag model and out past 450 yards the accuracy starts to slide down hill quickly. Some of the better programs such as Ohler still use the G1 drag model, but have massaged the mathematical calculations for better accuracy.
My ballistic program is near dead on to 800 yards for my .308. That is if I input all of the correct data. Beyond that it’s a matter of the me or the ballistic program as to what’s really correct, but were not off by much.
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Jeff
Mathew 5:16
Distance is not an issue, but the wind will make it interesting!

08242003, 02:29 PM

Platinum Member


Join Date: Jun 2001
Location: Palmer, Alaska
Posts: 2,539


Re: ballistic coefficients?
Max,
I may have missed the article and I'm not sure what drag curve Rick was refering to when he said the Scirocco was near a 1.0 BC but, if it was on the G1 curve.... he's all wet to say the least. I have the Oehler 43 data that says it's .560  .585 BC, measured at 300 yards with an acoustic target. This is the most accurate BC I've gotten from this bullet using the 43 PBL.
The problem with measuring BC's with the Oehler 43 and acoustic target, which Rick probably used in his tests, is that it calculates the downrange velocity based on time of flight, and with that it calculates the BC.
The problem with TOF measurments is if the inputed distance to the target was off by more than a tenth of a foot or so, it simply gives meaningless data. Even using a steel tape and very carefull measuring, it's possible to be off a tenth or two in the distance required to get accurate BC numbers.
The better way is with two seperate chronograph rails calibrated together near the muzzle, then with one set down range and hooked to the 43 or a seperate 35P unit... or get a dead on zero at 100yds then measure the "actual" drop in inches at a given distance beyond that while recording MV on a "good" chronograph. Take this MV and drop info to a ballistic program and use the average MV and enviromental data you collected, then manipulate the BC number until the actual drop on the ballistic chart matches what you were getting at the range... that's the most accurate way.
If you used two chronographs to test, look for the downrange velocity to match what you were getting as you manipulate the BC number. Ideally the drop in inches and the downrange velocity should point you to the same BC and testing both at the same time would be best.
The BC that I remember matching the 180gr Scirocco, and also the 180 Ballistic Silvertip at 3200 fps and my actual "scope settings" of 22.5 moa at 1000 yards and 7.7 MOA at 500 yds was between .530 and .550, nowhere near 1.0...
Barnes bullets are the only ones that I've tested that produce quite a bit different BC's than advertised, and most are much lower in reality. The 168gr XLC was an exception and was real close to advertised.
Like Jeff said, once you get out past 500  600 yards, the G1 curve may not be the best drag curve to use, and probably wasn't to begin with, but it will work to a point. Using it might even give you a different BC than the chrono downrange did. Why? The G1 drag curve not matching your actual trajectory curve. If the middle ranges don't match the trajectory curve your MV and new found accurate BC indicate, you must use a G5 or a G7 curve, which ever fits your "actual" fired drops best. Do yourself a favor and fire at least a five shot group at each distance and use the center of each group for the referance data point to match the curve to, a single shot at each 100 yard distance or whatever you decide on tells you absolutely nothing.
Jeff,
It's my understanding that a 1" artillery round with a 1.0 BC was the referance standard for the G1 curve, not a 45 ACP?
Oehler's Ballistic Explorer program does give the option to use multiple BC's at different ranges to duplicate your "actual" fired drop chart, as does as Exbal. It also has G1, G5, G6, G7, Gi, Gs and Ra4 drag curve options.
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Brent Moffitt

08242003, 03:33 PM

Silver Member


Join Date: Aug 2003
Location: McKinney TX
Posts: 408


Re: ballistic coefficients?
Brent,
You may be right on the 1" artillery round being the foundation for the G1 drag model. I've got a half dozen articles on BC's that I've saved over the years and each has there own basis for a starting point.
I use the RSI Ballistic program. I've found it superior to any other program on the market as long as I do my part and input the data correctly.
I shoot at TacPro here in TX, which is 1000yard range. I keep my actual data and compare it to the program. It's so close it's scary. I'm extremely confident when I go on the road for a hunt or competition that when I make my environmental changes, I'm pretty close to dead on at distances out to 750 and 800 yards.
I'm starting the range work up on my .308, which I just had rebarreled with a Mike Rock barrel. Initial results from the first 20 rounds of Hornady 168 AMAX ammo was 2 5shot groups at 100 yards in the mid .3's and low .4's. Not to bad starting out.
Have a great weekend.
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Jeff
Mathew 5:16
Distance is not an issue, but the wind will make it interesting!

08242003, 05:58 PM

Bronze Member


Join Date: May 2003
Location: Townsville Australia
Posts: 98


Re: ballistic coefficients?
cdixon, After shooting some 12,000 7mm 168 smks at Long Range Benchrest since 1997 i do not have an exact BC but only a guess.It's about .550,my old heavy/gun with a short 7mm mag was 3060 fps ,the come up from 100 to 1000yds was approx 23 minutes on the Nightforce "on my range".After talking to GTB bullets who incidently still makes the 168)this was a coupla years ago) aggreed with me on the .550 .This was checked at Tubbs range,also Larry Bartholome's real world test put this bullet at .510 @2615 fps and .544 @1517fps this is after a start of at 2800fps.The best part of this bullet is it is just as accurate at 2700 fps as it is at 3200fps.They respond best to bearing surface batching..JR

08252003, 12:17 PM

Bronze Member


Join Date: Oct 2001
Location: NC
Posts: 48


Re: ballistic coefficients?
i have been using sierras ballistic program
and im not sure what drag model it uses..
what do you (anyones opinion)think is the best ballistic software to use with ones own calculated b.c of a given bullet? also how would one calculate the b.c for a diffrent drag model? assuming that all given b.c's are for a G1 drag model..



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