New #2231 200gr SMK for F/TR
- Thread starter Quickoz
- Start date
avidflyer
Gold $$ Contributor
still waiting for the price..would like to try vs 200x
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.
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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.
— 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.
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Good catch titch—, I corrected my post.
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?
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.
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.
Why do Sierra still advertise the outdated G1 BC? Are they really convinced to fool shooters with higher G1?
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