Check my math/ theory please

Discussion in 'Reloading' started by texan79, Nov 19, 2010.

  1. texan79

    texan79 Well-Known Member

    Mar 28, 2010
    Here's the data: 7mm Rem Mag, 180grn VLD's, 68gr H1000

    Now, I dont own a chronograph, so I'm working with Berger's load data and trying to approximate my velocity. In order to achieve the desired energy on impact at 1000 yards, Exbal tells me I need to push this bullet at 2950 fps, so that is my goal.

    Here's what I'm working with and my thought process. Per Berger load data in a 24" barrel:

    66 gr= 2830
    62.5gr= 2672

    Difference of 3.5 grains of powder and 158 fps. Simple math says that 45.143 fps increase with each grain of powder. Using the 66 grains= 2830 as a baseline, I added 90 fps for the 2 grain increase in charge and got 2920 fps. I also have a 26" barrel and figure that I gain about 50 fps over this data.

    Therefore, it is my opinion that I am close to where I need to be with this load, energy wise.

    Can ya'll confirm or deny this theory/ math for me?

  2. edge

    edge Well-Known Member

    Nov 4, 2005
    Can you get that much powder in the case?

    What is the water capacity of your cases and how long can you make the loaded round?


  3. Kevin Thomas

    Kevin Thomas Well-Known Member

    Feb 16, 2009

    Sorry, but no, it can't be done. While we might be able to make a SWAG type guesstimate, that's all it is. You need a chronograph, or at least access to one. You're dealing with some calculations here that need precise data as a starting point, or everything else downrange is going to be purely coincidental.

    Got to be some other shooters in your area that would give you soem time on the chronographs, isn't there?
  4. texan79

    texan79 Well-Known Member

    Mar 28, 2010

    Yes I can. I'm loading that one .009 of the lands and getting .5 at 200. theres room for more, but I'm happy with the accuracy where I'm at.


    I figured it was a mathmatical longshot, but seemed logical to me. I'll have to catch a chrony at the range, or wait till after the holidays to pick one up. Thanks for the info.
  5. X3MHunter

    X3MHunter Well-Known Member

    Jul 8, 2009

    It doesn't work that way. You will need a chronograph, but if I were you, I would worry about accuracy before I worried about energy, you have to hit it before you can kill it!

    The other way to do it is if you have access to a range where you can shoot different distances and record your drops, then you can play with the speed and BC on exbal until your drops match.

    You are not far from Bayou Rifle Club, it's a great club and they have ranges from 100-1000 yards.
  6. jwp475

    jwp475 Well-Known Member

    Feb 4, 2005

    I'd forget about energy and worry about consistent acuracy. FPE is a very poor way to rate leathality.

    Kevin Thomas is absolutely correct. There is no way in hell to accurately estimate velocity from one gun to another even with the same load. A chronograph is the only way to KNOW
  7. J E Custom

    J E Custom Well-Known Member

    Jul 29, 2004
    The chronagraph is the best way to examine not only the velocity but also SDs,ESs,average
    velocity, highest and lowest velocity.

    The only other way I know to get velocity is to shoot your load over the coarse (100, 200 , 300 ,
    400 , 500 , 600, and if you have the range on up to 1000 yards) with at least 5 shot groups
    at each distance to get a good center of group and match it to ballistics tables with your bullet.

    When you match all distances with your trajectory then you will have your muzzle velocity.

    This was/is also a good way to get a drop chart and an accurate BC for your bullet.

    All rifles are different and math just won't get you there.

  8. Nalgi

    Nalgi Active Member

    Oct 23, 2010
    Re: Just borrow a Chrony!

    then the math will be perfect~
  9. Loner

    Loner Well-Known Member

    Apr 14, 2010
    2 boxes of those bergers will buy you a chrony. It'll be a whole lot cheaper in the long