7MM RUM/WC872 and the chrony ...

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by dwm, Jun 12, 2003.

  1. dwm

    dwm Well-Known Member

    Messages:
    739
    Joined:
    Feb 26, 2003
    Hey guys, I have some data here that doesn't quite make sense.

    At the range yesterday I shot 5 - 5 shot groups with the 7MM RUM, 28 inch barrel, not counting muzzle break.

    Here is the data:

    New cases, full length sized with Redding FL bushing die (0.310). Cases neck turned to 0.013 thickness. Chamber has 0.312 neck.

    Fed GM215M primers
    WC872 powder, all charges weighed to +0.1 grains
    Nosler 160 Accubond bullets

    Temp: 91 degrees, 60-70% humidity, close to sea level.
    Wind: 3-5 gusting to 10, straight into bench

    Six fouling shots fired before this data was collected.

    Barrel was allowed time to cool between each shot.

    Group 1 - 98 grains (2 1/2 inch group)
    3446
    3416
    3474
    3458
    3486

    Group 2 - 98.5 grains (2 1/2 inch group)
    3492
    3567
    3502
    3454
    3489

    Group 3 - 99 grains (2 1/2 inch group)
    3466
    3539
    3429
    3541
    3491

    Group 4 - 99.5 grains (3 within 1/2 inch, other two low 1 inch in different directions.)
    3484
    3555
    3454
    3470
    3452

    Group 5 - 100 grains (2 inch group, 4 in the corners of 2 inches)
    3483
    3482
    3482
    3486
    3510

    No pressure signs at all, no marks on any cases, no sticky anything.

    Why didn't the velocity change more?

    Is WC872 really that slow burning that a 2 grain difference only made the velocity go up 30 feet per second?

    Am I using enough neck tension?

    Some not to good groups.

    Doug
     
  2. Guest

    Guest Guest

    Dwm...Two possibilities come to mind, light changing as the sun moves may cause different readings, and your barrel may be to short to show a significant velocity increase.

    Just as a reference, 4 additional grains of 50 BMG will cause an increase of 120 fps. in a 30 inch barrel in a 30 Wolf, when your base load is at 65,000 psi.
     
  3. steve smith

    steve smith Well-Known Member

    Messages:
    244
    Joined:
    Jul 4, 2001
    I wouldn't be to worried about the velocity increase. I found the same thing in my 30" barreled 270-300 RUM. I was using the 135MK with 98gr WC872, pretty much max load. The rifle has a .300" neck, all case necks turned to .0105" per side. Loaded round mics. out to .299" +.0"-.001". Anyhow, I started the load @92gr, If I remember correctly, and ended @103.5grs. The velocity started around 3600fps at 92gr, jumped up to 3680fps by the 96gr mark, by the time I hit 103.5gr the velocity was just over 3800fps. in the first 4gr the vel. increased 80fps, but in the last 7.5gr it increased 120fps. The problem is with the case capacity, to the brim of the neck is around 113gr. of WC872. You and I both are in the realm of extremely overbore rifles. We are pushing small projectiles with very large amounts of propellant at relatively fast velocities. The more capacity you add the less effect the added capacity has. Where as an additional 2gr. in a 270win might give you an extra 100+fps, but the same additional 2gr in my rifle hardly even shows a difference. It took 11.5gr to make 200fps difference. But the velocity was a little more predictable when using 5010. I've achieved the earlier mentioned velocity of 3800fps @97gr, and much better accuracy. With wc 872 my best groups weren't much better than 1-1.5". with 5010 the groups dropped to below 1".

    [ 06-17-2003: Message edited by: txhunter ]
     
  4. Mark_in_utah

    Mark_in_utah Well-Known Member

    Messages:
    71
    Joined:
    Jul 30, 2003
    DWM,

    You're in the realm of VERY small load changes. While 0.5 grains SEEMS like a lot, it's about 1/2% load change. 1/2 a grain in smaller calibers is more like a full 1% load change. You're also into the load efficiency thing, where the powder is less efficient at pushing the bullet down the barrel.

    I agree with what was said, you can't expect the velocity to change much with 1/2 grain load change in this cartridge. I've seen the same thing in my .338 RUM, and I'm just starting my load development with it.

    Mark in Utah