How is this possible?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by chas3stix, Oct 2, 2011.

  1. chas3stix

    chas3stix Well-Known Member

    Messages:
    207
    Joined:
    Jun 7, 2011
    I was visiting the Accurate Shooter site and came across info on the 30BR round.
    The parent case is either 6mm or7mmBR. The case is necked up for 30 caliber bullets.
    Here's the part I don't understand.....they're claiming 2900 to 3000 fps with the 30BR using33 Gr. of H- 4198 powder. For a comparable load with the 7mmBR, the maximum velocity is 2685 using 28.5Gr. of H-4198. (both rounds shooting a 115Gr.bullet). So how do they get the extra fps?
    I'm no ballistician or math whiz....would appreciate someone out there to explain this to me.Thanks in advance.
    Chas
     
  2. backwoods83

    backwoods83 Well-Known Member

    Messages:
    2,206
    Joined:
    Jan 8, 2011
    Well, being that the 7br is .284" diameter bullet and the 30br is .308 and they are both likely flat base bullets so 2 simple reasons are 30 cal has a larger base to push against and a much shorter bearing surface, which leads to less pressure in the 30 cal with equal powder charges, so you can use more powder and get more speed.
     

  3. liltank

    liltank Well-Known Member

    Messages:
    4,158
    Joined:
    Nov 3, 2008
    Faster powder will work better due to the larger throat diameter too.

    Tank
     
  4. Mikecr

    Mikecr Well-Known Member

    Messages:
    4,249
    Joined:
    Aug 10, 2003
    +1 Backwoods
    And 4.5gr more powder..
     
  5. chas3stix

    chas3stix Well-Known Member

    Messages:
    207
    Joined:
    Jun 7, 2011
    Thanks for the explanation.
     
  6. RT2506

    RT2506 Well-Known Member

    Messages:
    1,498
    Joined:
    Jan 10, 2008
    I think they are using a pretty fast twist barrel also.
     
  7. liltank

    liltank Well-Known Member

    Messages:
    4,158
    Joined:
    Nov 3, 2008
    They state a 1:14 or 1:15 twist. That will work up to about a 125grn bullet.

    Tank
     
  8. Mikecr

    Mikecr Well-Known Member

    Messages:
    4,249
    Joined:
    Aug 10, 2003
    Yeah they use very slow twists(out to 18tw), which is a great contributor to their performance.
     
  9. KRP

    KRP Well-Known Member

    Messages:
    591
    Joined:
    Dec 25, 2007
    "Tactical" guys might be, shooting heavy bullets at subsonic velocities through suppressors, but the Accurate Shooter guys are probably shooting Benchrest for score with slow twist barrels and light bullets.
     
  10. Trickymissfit

    Trickymissfit Well-Known Member

    Messages:
    4,071
    Joined:
    Jun 11, 2010
    actually the big rounds with a 30BR are the 115 thru 120grain in a 16 or 17 twist barrel. Pay a visit to the BIB Bullet website. The 17 twist is a little temperature sensitive, and probably the 16 would be better in cool weather
    gary
     
  11. vendetta333

    vendetta333 Well-Known Member

    Messages:
    199
    Joined:
    Jul 2, 2011
    with ballistics like that, y does this not sound like a great hunting round to anyone? sounds to me like a little under 243 to 260 performance but with waay less powder! sorry if i sound like a tard. plz correct me. be gentle lol lightbulb
     
  12. liltank

    liltank Well-Known Member

    Messages:
    4,158
    Joined:
    Nov 3, 2008
    I really considered it as such. The problem is getting decent hunting bullets that are non varmint in those weights.

    Tank
     
  13. backwoods83

    backwoods83 Well-Known Member

    Messages:
    2,206
    Joined:
    Jan 8, 2011
    As mentioned these 115 and 120grn pills like the BIBS are flat base, no boattail, the bc is not that good and they are used for 1, 2, and 300yrd benchrest where a high bc bullet is not a necessity, that's why they are not good hunting rounds, now a 8 twist 6br with 105 bergers is a good whitetail/antelope stopper.
     
    Last edited: Oct 4, 2011
  14. vendetta333

    vendetta333 Well-Known Member

    Messages:
    199
    Joined:
    Jul 2, 2011
    well, i was thinking as a great <300yrd deer rifle, but i did not think about bullet construction, silly me, you're right they are mostly varmint bullets. glad i wasn't totally off though. :D