New #2231 200gr SMK for F/TR

Discussion in 'Big Stuff--7mm, 30 Cal, .338+' started by Quickoz, Jan 18, 2018.

  1. Quickoz

    Quickoz

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    G'day guys,

    Who is going to be the first to try the new 200gr SMK for F/TR?

    I have a 10T coming and will try them. I do know that they need a 9T from what Sierra says.

    Regards,

    Shaun
     
  2. Zero333

    Zero333 Silver $$ Contributor

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    I shoot 240 smk in a 300winmag with a 10" twist. So can't see why a 9" twist will work better than a 10" twist with the new 200 smk. Regardless of what Sierra claims.
    Edit: I see on Sierra's site they recommend a 9" twist but my opinion stands.

    I know you're talking about 308win.
     
  3. avidflyer

    avidflyer Silver $$ Contributor

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    still waiting for the price..would like to try vs 200x
     
  4. JBT

    JBT

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    I’m sure I will try them in a .300 Win Mag and a WSM at some point. Not in any hurry though. I shoot 230 Hybrids through my 10 twist and they perform great out past a mile so the 200 should be stabilized out of a 10 twist.
     
  5. Bart B.

    Bart B.

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    I've asked Sierra to list twist rates for small and large cartridges for their bullets. That new 200 grain bullet may need a 1:9 twist 308 Win barrel. But a 1:11 would do for 30 caliber large magnum cases
     
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  6. titch--

    titch--

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    Im looking at trying these out too, but I want a new barrel and wondering what kind of freebore would be good for them? Would they be similar with a .170 that the 200.20's like?
     
  7. Bart B.

    Bart B.

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    How does freebore length effect the accuracy any bullet might have?

    It's zero in SAAMI spec 30-06 and 300 Win Mag chambers, over a quarter inch in 300 WSM chambers. Other 30 caliber chambers are in between. With the right barrel twist, they all should shoot the 200.20 equally excellent.
     
  8. titch--

    titch--

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    Its not really about accuracy, its more about the seating depth and how much powder you can get in your case so you can shoot these bigger bullets at a decent speed to take advantage of these big BC bullets. Thats my understanding.
     
  9. fyrewall

    fyrewall

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    How long is the subject Sierra bullet compared to the Berger 200 20 X bullet that is 1.51 inches long.

    I would guess that the Berger twist calculator uses the Miller twist formula . As the Berger calculator shows at a velocity of 2300 fps using a 1-10 twist, for their 1.51 inch long 200 20X .308 bullet, the Sg will drop below 1.5, like 1.49 making it "marginally stable". Using a 1-9 twist the 2300 fps Sg is 1.84. Using a .308 Win 1-10 twist at over 2400 fps the Sg should be over 1.5.

    The new Sierra bullet may be longer than 1.51 inches. I would not select that bullet to shoot out of my 1-10 twist .308 Win rifle on a cold day.

    Other conditions affect the Sg values obtained with the Miller formula like caliber, length, and temperature being the most important. The Miller business uses fractional values calculated from comparisons of actual values with standard values and velocity appears to be one of the least significant. Some standard Miller values are: 2800 fps, 29.92 atmospheric pressure, 59 degrees F and - 460 F. Running the Berger twist calculator for selected values using the same bullet will show velocity to be not that important a factor in Sg calculations. Look at changes of Sg when changing velocities 100 fps - they are minor like .02 - .03 or so.
     
    Last edited: Jan 20, 2018
  10. 6ShotsOr5?

    6ShotsOr5? Gold $$ Contributor

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    If you look at Sierra’s picture on the .308 caliber bullets page, it looks like the 2231 Match King at 200 gr is a bit longer than the old 240 gr Match King, and the 240 gr is 1.591” Long. At sea level, the bullet will need a 9 twist down at 2500 fps, I don’t think you can push it fast enough to make it stable with an 11 twist barrel:. 9.5 or 9.75 may be as low as you would want to go and that’s up above 3000 fps.

    — EDIT — Thanks to Titch — I need to take out my contacts when reading fine print. The new 200 is shorter than the old 240, but longer than the 210 and 220. Call it a 1.54” length, then I get 1.44 corrected Miller with a 10 twist at 2500 fps muzzle so a 308 shooter could benefit a little from faster twist but the 10 twist should be fine for 300 Win Mag or higher MV’s

    To get the BC Sierra is claiming, the new bullet needs a form factor of around 0.85. This is a better form factor than any bullets in Litz’ Ballistic Performance of Rifle Bullets 2nd Edition until you get up to some of the .338 size and larger bullets. None of the 7mm / .264 cal bullets have a form factor this low, but the new SMK 230 has an even lower form factor.
     
    Last edited: Jan 19, 2018
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  11. titch--

    titch--

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  12. 6ShotsOr5?

    6ShotsOr5? Gold $$ Contributor

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    Good catch titch—, I corrected my post.
     
  13. FatBoy

    FatBoy

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    this may be nothing more than my mind wandering, or may be common knowledge I haven't heard , but it seems every time a bullet maker releases a bullet with a shorter bearing surface they increase the twist rate. Is this just due to the shape of the bullets in front of the lead bearing edge or do they need the extra twist just becuase they aren't purchasing the lands as much as an older, heavier bullet?
     
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  14. JBT

    JBT

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    I also wonder about the short bearing surface and how it will perform in a 5R land profile vs 4 groove.
     
  15. Zero333

    Zero333 Silver $$ Contributor

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    They're trying to maximize the BC so they need to make the nose as long as possible, making the bullet longer and bearing surface shorter.
    That material has to come from somewhere with out increasing the weight of the bullet. Hence the shorter bearing surface.

    How much bearing surface is on the bullet has less to do with the optimal twist race than the length of the bullet.

    Think of a football spinning in the air when thinking about stabilizing bullets.
     
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  16. Bart B.

    Bart B.

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    Yes, more weight forward from where the ogive starts.

    A given weight bullet made longer requires its bearing surface be shorter. That extra length has to get metal from some place. Longer bullets need faster twists.
     
  17. FatBoy

    FatBoy

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    I understand the concept of squeezing the ballon. Question is, why the added twist? the length of the bullet or the surface area of the bearing surface?

    Maybe I should start another thread?
     
  18. Bart B.

    Bart B.

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    Length of bullet. Bearing surface length is not an input to stability calculating formulas.

    Bullets are going a few dozen fps entering the rifling. Rifling angle is a few degrees.
     
  19. Ilya S.

    Ilya S.

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    Anybody knows their G7 bc and correct length ? I wanna table to compare with 20x.
     
  20. Marksman63

    Marksman63

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    Why do Sierra still advertise the outdated G1 BC? Are they really convinced to fool shooters with higher G1?
     
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