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Actual B.C.

 
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  #1  
Old 02-12-2007, 11:39 PM
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Actual B.C.

Can you figure actual B.C. with just muzzle velocity and bullet drop at certain ranges? If so, what ranges do you need to check the bullet drop? I am going to have Leupold build me a BDC elevation turret for one of my guns and I wanted to provide them with an acurrate B.C.

Thanks for all the wisdom,

Hank
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  #2  
Old 02-13-2007, 12:20 AM
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Re: Actual B.C.

Well from what I understand, you can match your trajectory with a certain muzzle velocity to obtain BC verified by drop at various distances. You'll get a better idea of BC the farther you shoot. For instance my 7 RM and 140g NBT. Velocity over my CRONY reads 3250fps but I begin to wonder. With a 100y zero, it should drop right at 20" at 400 yards. Well they were dropping 23". The BC of the 140g NBT is listed at .485 which seems alright. Now for me to actually be dropping that much, my BC would ahve to be around .350, I doubt it, or my muzzle velocity is 3075fps. You kinda just have to shoot and play around to figure things out, but thas what makes this sport so fun, or frustrating, either way you look at it.
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Old 02-13-2007, 12:24 AM
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Re: Actual B.C.

[ QUOTE ]
I am going to have Leupold build me a BDC elevation turret for one of my guns and I wanted to provide them with an acurrate B.C.


[/ QUOTE ]

why not give them the actual drops, the custom shop will make them that way as well.

d-a
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  #4  
Old 02-13-2007, 03:34 PM
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Re: Actual B.C.

That is a good point to a degree. I won't have the opportunity to shoot at all ranges right now, but I would like to have the values correct for other distances other than the ones I am able to shoot.
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  #5  
Old 02-13-2007, 06:16 PM
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Re: Actual B.C.

What bullet....I do have some magazine articles that have much different results than manufacturers claims. I know someone somewhere did some chrono work at 300 yds with start/finish speeds (brave guy) fairly recently but I'll be darned if I can find what forum.

What bullet are you interested in??
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  #6  
Old 02-13-2007, 07:19 PM
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Location: The rifle range, or archery range or behind the computer in Alaska
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Re: Actual B.C.

[ QUOTE ]
Can you figure actual B.C. with just muzzle velocity and bullet drop at certain ranges?

[/ QUOTE ]


In short, yes. There is more to it than just that. You will need a very accurate or very close muzzle velocity as you have stated. Also you will need a VERY accurate zero. Preferably 200 or even better 300 yards to "iron" out the small things when dealing with a 100 yard zero. I will explain later. Next you will need precise atmosheric conditions. Temp, humidity, pressure. Then you fire your loads in 100 yard increments out to the maximum distance you will shoot tagets and/or game. Dont just go to your max range and call it good. I will also explain later. Then you can go to your ballistics program and input all the known variables. Next step is to adjust you drag model and BC untill the trajectory output is the same or as close as possible to your real drops.

Now the fun part. To figure out how many inches your bullets droped from zero, you can use the click adjustments or hold overs and do reverse math. For example, if it takes 100 clicks to hit dead on at 1000 yards from a 300 yard zero, and your using a true .25 MOA per click scope, your bullet dropped 261.75 inches from zero. You can convert holdovers as well.

Now on to explaining the longer zero. First there is nothing wrong with using a closer zero for general use. This is because you have spent time with the setup and know how it works and is of no consequense. When you are trying to find a true BC, you need more precision. The reason is that if your 100 yard zero is off 0.25" at 100 yards, this is typically an accecptable zero. But, and thats a heavy butt, that .25" makes for alot of error at 600 or 1K. A .25" error with a 300 yard zero is only a very slight issue at 600-1K. So for finding a true BC, start at 300 yards and go from there. Then if you like, go back to 100 for general use.

Next topic to be explained. The reason you shouldnt go straight to your max range is because you can arrive at your max range with the same drop for several differant loads with vastly differant mid point trajectories. This is where using the correct drag model and BC come into play. There are many BC/drag combos that will give the same end result but differant mid point results. So in effect, you could miss a target at 750 yards because you didnt verify you mid point trajectory even though you would drill it at 1K.

As has been mentioned, you could just give the turret maker your drop info!
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  #7  
Old 02-13-2007, 11:02 PM
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Re: Actual B.C.

Thanks for all the info guys! My 6.5-300WBY is giving me up to 3400fps with 140 gr. bullets. I will decide what to shoot when I can give them a try at 300 yds. I am looking at the SST, SMK and SGK. Sierra gives different B.C. for different velocities. I should have faith in Leupold to get the BDC turrets right. What are your thoughts?

Hank
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