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Help with BC calculation

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Unread 08-16-2004, 03:46 PM
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Join Date: Jul 2001
Posts: 1,459
Help with BC calculation

Just finished with my drop table of my 6.5 and 140gr Hornady SST bullets.

Would appreciate if others could verify the BC based on the following data.

Muzzle vel is 2975fps 10ft from muzzle.
drop/come up at 800yds 15 min, 700yds 11min, 600yds 8min, 500yds 6min, 400yds 4min, 300yds 2.5min.

Ranges verified with a Leica 800. Come ups based on Elite 4200 6X24 turrents. Come ups were repeatable. Amount per click has been verified at 100yds.

All drops are based on a 100yd zero.

From the program I am using, I am getting some very high BC's. Would love to see what your program says. Conditions are sea level, 85 F, 30% humidity.

Appreciate the help...

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Unread 08-16-2004, 05:36 PM
Platinum Member
Join Date: May 2001
Posts: 2,369
Re: Help with BC calculation

Something is ODD!

Are you sure about the speed, 2975fps???

The BC would be over .85 and I still can't get a trajectory match.
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Unread 08-16-2004, 09:02 PM
Bronze Member
Join Date: Oct 2003
Location: Randolph
Posts: 57
Re: Help with BC calculation

I also tried running the numbers with a BC of 1.25 and thats still to low. I also tried running the numbers shooting at an angle. They came out closer with a 30-35 degree shooting angle, but only work from 300-600 or from 600-800 and there still not right on. Something's got to be wrong????

Just ran the numbers again with a MV of 3350 and the numbers come out much closer. Also tried the match the trajectory using your come ups, and exbal calculates an MV of 3401 with using a BC of .512 but the 800 yd comes up shy at 13.5

I'm just learning all the tools of this Exbal software, (got it last week). I'd have to say thats its a pretty good program.

Probly didn't help, but I tried [img]images/icons/smile.gif[/img] Good shootin

[ 08-16-2004: Message edited by: Cobber ]
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Unread 08-16-2004, 09:02 PM
Gold Member
Join Date: Jan 2002
Location: Wenatchee, WA
Posts: 806
Re: Help with BC calculation


It would be helpful if you could tell us what your scope height above the bore is... using JBM on the web, it appears to make a significant difference. Me thinks your chrono may be b0rked? That's the only way I could come close to your numbers:

Trajectory (Basic) Output


Input Data
Muzzle Velocity: 3250.0 ft/sec
Ballistic Coefficient: 0.520
Drag Function: G1
Bullet Weight: 140 grains
Sight Height: 2.50 inches
Wind Cross Speed: 10 mph
LOS Angle: 0 degrees
Target Speed: 10 mph
Zero Range: 100 yards
Temperature: 85.0 F
Barometric Pressure: 29.50 in Hg
Relative Humidity: 30.0 %
Altitude: 0 feet
Air Density: 93 % of Sea Level


Calculated Table
Elevation: 4.039 moa
Azimuth: 0.000 moa

Range Velocity Energy Momentum Drop Windage Lead Time
(yards) (ft/sec) (ft-lbs) (lbs-sec) (moa) (moa) (moa) (sec)
100 3063.7 2917.8 1.90 -0.0 0.5 16.0 0.095
200 2884.8 2587.0 1.79 -0.6 1.0 16.5 0.196
300 2712.9 2287.8 1.69 -2.1 1.5 17.0 0.303
400 2547.6 2017.5 1.58 -4.0 2.0 17.5 0.417
500 2388.8 1773.9 1.49 -6.2 2.6 18.1 0.539
600 2236.0 1554.1 1.39 -8.6 3.2 18.7 0.669
700 2088.1 1355.3 1.30 -11.2 3.9 19.4 0.808
800 1946.1 1177.3 1.21 -14.2 4.6 20.1 0.957
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Unread 08-17-2004, 03:00 PM
Posts: n/a
Re: Help with BC calculation

Did you guys ever agree on the flight numbers? I'm going to need some help with my 6.5 264 WinMag. Can get you whatever numbers you need for factors. Thanks!
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Unread 08-18-2004, 02:45 PM
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Join Date: Jul 2001
Posts: 1,459
Re: Help with BC calculation

Jeff, I would have to say that my velocity is pretty much right on. Reason is simply this is not a new wildcat and there have been lots of shooters who have achieved this type of velocity. They have never achieved 3100 to 3200fps with a 24" barrel without blowing themselves to kingdom come.

Pretty much impossible too with so little powder. In line with performance from its closest cousin, the 270 Win.

Also, I have used this Chrony with other rifles and they have all "agreed".

Now if the chrony cannot be trusted at 10ft. Why would I trust its readout at 100yds?

So the only error possible is that the chrony is reading too fast. If that were the case, the BC would be even HIGHER. Now, that would be seriously screwy.

I have attached a pic in another post (my new 6.5 toy) so that you can see that this is a very standard rifle and scope set up. Scope height is pretty much 1.5", etc.

My suggestion is still that any 6.5 shooter get some of these SST's and shoot them. At 2975fps, do you get the same or similar drop?

The correct BC may or may not be the 0.9 or 1 but it certainly is above 0.6. Either way, this is a bullet worth further investigation.

Capable of shooting in the 0.1's too.

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Unread 08-18-2004, 05:11 PM
Silver Member
Join Date: Aug 2003
Location: McKinney TX
Posts: 408
Re: Help with BC calculation


Sorry I'm not questioning your results only trying to understand them better. This stuff ain't rocket science, but there are the laws of physics and is usually pretty straight forward.

I've never had much luck with my two chrony's. But getting an average velocity readings at exactly 100 yards and having your average muzzle velocity will establish your correct BC and then we can use the different drag models to better fit it to your results.

Just trying to help. There are a lot of explanations for your results, but having exact numbers help.


[ 08-18-2004: Message edited by: Jeff In TX ]

Mathew 5:16

Distance is not an issue, but the wind will make it interesting!
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