Actual B.C.

Discussion in 'Reloading' started by hank shaper, Feb 12, 2007.

  1. hank shaper

    hank shaper Well-Known Member

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    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
     
  2. remingtonman_25_06

    remingtonman_25_06 Well-Known Member

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    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.
     

  3. 1doug

    1doug Well-Known Member

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    [ 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
     
  4. hank shaper

    hank shaper Well-Known Member

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    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.
     
  5. kraky

    kraky Well-Known Member

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    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??
     
  6. Michael Eichele

    Michael Eichele Well-Known Member

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    [ 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!
     
  7. hank shaper

    hank shaper Well-Known Member

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    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
     
  8. goodgrouper

    goodgrouper Well-Known Member

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    My thoughts on it are this:

    6.5-WWH cartridge is hard enough on barrels that your bc will be degrading with just about every box of bullets you buy so no matter how accurate your starting bc is that you give to Leupold, it will not stay that way for long. Then throw in a half mark (like 725 yards) then a little angle, then an unexpected elevation change, and your little tidy mark on the turret is suddenly WAY off.

    If you can't already tell, I am very much anti BDC and anti holdover dot for long range hunting. /ubbthreads/images/graemlins/shocked.gif /ubbthreads/images/graemlins/frown.gif Yes, they might work for mid range, but I absolutely positively guarantee you that you will be let down hard eventually at long range with either of these systems. People who disagree with me on this one have either not had the let down happen yet or have not shot at a big game animal past 800 yards.

    Do yourself a favor. Learn MOA and IPHY and use real world input data from your current hunting situation. Then dial your scope in MOA (or IPHY) for your current bc. It will be simple, you will still be dialing to a number, it's just that the number represents a different thing. And best of all, if you learn MOA, you can transfer this techinque to all your guns because their scopes are all set up in this manner.

    Sorry John. /ubbthreads/images/graemlins/frown.gif
     
  9. Michael Eichele

    Michael Eichele Well-Known Member

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    [ QUOTE ]
    and your little tidy mark on the turret is suddenly WAY off.


    [/ QUOTE ]

    That is why I dont use pre-set marks on my turrets. For me, every hunting situation is differant. Differant inclines, atmosperic conditions and differant altitudes. I calculate each shot for what it is and adjust accordingly. However, if my ballistics program tells me that it just so happens to be that for my 875 yard shot I need 8.0 or 8.1 bars on my NP-R2 reticle to hit my target for this particular set of circumstances, I will use my 8th bar instead of clicking my way to zero. If it isnt very close to a whole number, I will go ahead and come up in MOA on my turret.
     
  10. hank shaper

    hank shaper Well-Known Member

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    Thanks Good Grouper and I can appreciate what you are telling me. Do I understand you right, due to barrel erosion, velocities change. This then changes the B.C. Therefore, the BDC values will be wrong as the barrels wears? The same thing will happen to my drop chart also, but it is easier to correct, right? How accurate do you guys find the computer generated drop charts to be? How do I go about making one?

    What do you think my barrel life will be running 140 gr. bullets at 3350-3400?

    Hank
     
  11. Michael Eichele

    Michael Eichele Well-Known Member

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    I know I am not GG, but yes, velocity can change due due barrel wear. Also due to barrel wear the engraving on the bullet changes which can change the BC also. Both velocity differances and less quality engravings change the BC. Both together and alone. Now we are talking about minor changes here. You wouldnt notice much differance out to 600 yards.
     
  12. goodgrouper

    goodgrouper Well-Known Member

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    [ QUOTE ]
    Do I understand you right, due to barrel erosion, velocities change.

    [/ QUOTE ]


    Yes. They can either increase or decrease.


    [ QUOTE ]
    This then changes the B.C.

    [/ QUOTE ]

    Yes, but only slightly. The main thing is that the bc changes because of the wear.


    [ QUOTE ]
    Therefore, the BDC values will be wrong as the barrels wears?

    [/ QUOTE ]

    Yes, absolutely.



    [ QUOTE ]
    The same thing will happen to my drop chart also, but it is easier to correct, right?

    [/ QUOTE ]

    Yes. You will constantly be checking your bc values and will run your charts or PDA program accordingly on the hunt or within days of being on it. So it is much easier to hit "calculate" on your program or print a new page off your home pc then it is to have a new turret cap engraved.




    [ QUOTE ]
    How accurate do you guys find the computer generated drop charts to be?

    [/ QUOTE ]

    Programs are only as accurate as the data they are given to compute. The more you know about where you are and what the conditions are, the more accurate the program will be. Given the right information, most programs will predict where your bullet will be in it's trajectory to the 1/2" as far out as you want to know.




    [ QUOTE ]
    How do I go about making one?



    [/ QUOTE ]

    Get yourself a copy of Exbal, run the drops in IPHY, and then make sure you have a good printer.




    [ QUOTE ]
    What do you think my barrel life will be running 140 gr. bullets at 3350-3400?



    [/ QUOTE ]

    For what size target? You will probably still hit the side of an F350 at 1000 yards after 1500 rounds, but you might not be able to dust a p-dog at 1000 yards after 500 rounds.

    A buddy of mine has a 6.5 wsm (which is smaller than your case) and just had it borescoped the other day and it looked like a dry lake bed 6 inches down the bore from the chamber and he has less than 500 rounds down it. Now, it still shoots ok, but it is getting noticeably harder to clean and it is now blowing up every bullet he shoots except for the 142 grain Sierra MK. They have a tougher jacket than the Bergers and Amax.

    And the MK's no longer use the same chart he started with!


    Eventually, when your barrel does not give you the accuracy you want and is a #$%*@ to clean, you can run a Tubb's barrel maintenance kit through it and revive it for ahwile but in the meantime, you would have gone through about a dozen BDC knobs trying to keep up with your ever-changing barrel.