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Velocity and BC?

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Unread 10-01-2005, 08:12 PM
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Join Date: Jul 2001
Posts: 1,459
Re: Velocity and BC?

A couple of ways to check. At 100yds, put up a tape measure on a board. Zero the rifle at the bottom (shoot a rd). Dial up the scope and shoot at the different come ups. See where the bullets actually land.

If the scope is off, you will quickly see. Example, you dial up 10 min but your actual bullet impacts 12 min high. The scope moves more then expected so it looks like your bullet is flying flatter then it actually does.

If all the come ups check out, shoot at your favorite distances but on a MOA sized target. Shooting and hitting a 40" rock does allow for some degree of error (2MOA or more). If you shoot and hit a 11" target at 1100yds, well then you would have to say things are dialed in.

Finally, what drag function are you using in your data. That is the G number usually required. Most programs default to G1 which is usually wrong for boattail bullets.

If everything else checks out, change the G function but keep all other data the same. It might bring your data in line with real world observations.

That is what I had to do with recent shooting of the 162gr Amax from 2 7RM. By changing the G function, I was able to get my data and real world come ups to agree.

Hope this helps. Try and get yourself a chronie. Would reduce the amount of guesswork.

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Unread 10-01-2005, 09:45 PM
Platinum Member
Join Date: Aug 2003
Location: NC, oceanfront
Posts: 3,977
Re: Velocity and BC?

What scope are you using?

Peja's utilizes Ingall's retardation model. But checking with other software, your 5moa 500yd isn't right. G7 BC correction doesn't help either.
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Unread 10-02-2005, 03:54 AM
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Join Date: Dec 2001
Location: Mukilteo, WA
Posts: 1,092
Re: Velocity and BC?

Try measuring the actual "click" value of your scope. I'm guessing your particular scope simply moves more than 1/4 MOA for each click.
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Unread 10-02-2005, 06:21 AM
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Join Date: Aug 2003
Location: McKinney TX
Posts: 408
Re: Velocity and BC?


If you have access to a ballistic computer you can usually reverse engineer your data. However, you need good input data. The BC that Sierra advertises is a baseline, your gun with your load and velocity may change it a bit.

When I see folks doing this, the biggest mistake in reverse engineering is the scope height. Most use a standard 1.5 inches that comes in the ballistic computers. You need to measure it and use the correct height.

Then once you have all the input data, start playing with the BC until it closely matches your actual data. You won't get it exact, but you should be able to get close.

Hope it helps,

Mathew 5:16

Distance is not an issue, but the wind will make it interesting!
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Unread 10-02-2005, 07:15 AM
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Join Date: Aug 2003
Location: NC, oceanfront
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Re: Velocity and BC?

Did you "zero" your turrets at 100yds?
You have to discount MOA needed for initial setup.
If you haven't, that's your problem.

I get a pretty good match using EXBAL(which is G1) when I set the zero to minimum for this program(25yds).
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Unread 10-02-2005, 07:45 AM
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Join Date: Jul 2003
Location: Central PA
Posts: 1,307
Re: Velocity and BC?

Wow, saw 5.25 to 500 in the first post, and knew sumpin' aint right [img]/ubbthreads/images/graemlins/shocked.gif[/img]

The only time this has happened to me is when i was shooting over a dip in the terrain -- about a 25-30ft drop between me and the target.

The wind was coming at me from about 1 'o clock and LIFTING the bullet when it hit the dip and flowed up and over.

Got any terrain like this between you and the 500yd target?

Check it Out!!--> Shoot the Smack The Smiley Match...and help
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Unread 10-08-2005, 03:53 PM
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Join Date: Dec 2003
Location: Utah
Posts: 70
Re: Velocity and BC?

I just thought I would update all that have replied to this post. First off, thank you for enlightening me to my dilema. Second, I have no idea what I was thinking. I posted this question w/o even giving a second thought as to what I should do.
I scratched my head and decided that something was WAAAAY wrong and loaded up the rifle and gear and went to the range. Well to make a long story short, My firt shot was 6" high at 100 yds. Need I go on?? I didn't think so. Re-zeroed the scope and wouldn't you know, my ballistic drop tables were right all the time.
Thank you all.
"And that law of the land which is constitutional, supporting that priciple of freedom in maintaining rights and privileges, belongs to ALL mankind..." -D&C 98:5-
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