Home Forums Reloading Varmint Rifles My gun is confused on twist rate

I am very confused, does anyone check .224 bullet lengths, velocity and barrel twists. Starting with 50 gr and 55 gr on the following; Hornet, 221 Fireball, .222 and .223. Any other of the .224 I did not check, but should be above 3400 fps. I have found different twist calculators. Some are very old and served us very well over the years. William F. Davis, Bowman-Howell, Bob McCoy, Greenhill, and the newest one by Don Miller. Of the course everyone states they have the best. That is not concerns me. For years I have used 55 gr Blitz Kings shot out of 1:12 Ruger KM 77VT MKII. The 55 gr I have chronograph between 3150 to 3250 fps depending on powder, temperature, ect, ect. Now I have never seen any problem with 55 gr. Blitz Kings and my .223 @ 3250 fps. Most talk about a “Stability Factor” should be above 1.2 up to 1.5. I have look for bullet lengths on the internet and found some to be incorrect. The best way is to measure at least 10 from you stock. I measured 55 gr Blitz Kings at .826. They will vary from lot to lot, but a couple of thousands will not matter. If I use William F. Davis with stability factor of 1.2 it requires 11.29 twist. If I use the Miller calculator it states the stability factory is .999. Miller uses red at below 1.0, yellow is above 1 up to 1.3 and green is above 1.3. Now if you use V-Max it is very near the same. If you are using a Hornet 1:14 twist 50 gr .784 length Blitz King at 2500 fps Miller shows .709 stability factor. With 221 Fireball that may have 1:12 or 1:14 with same 50 gr Blitz Kings at a velocity of 2900 fps Miller shows 1:12 @ 1.01 stability and 1:14 @ .745. It is very good that my Ruger cannot read twist rate calculators. Now I have a Fireball I did find that Sierra 50 gr Blitz # 1330 is .673 long, and will work with Fireball.

McGoo,

You apparently didn’t enter the local temperature and altitude in the Miller formula when calculating twist for the 55 BlitzKing. Both have an enormous effect on bullet stability, but the “default” setting in the formula is standard conditions, 59 degrees Fahrenheit at sea level. Plug in your typical local conditions and you’ll get a different result, probably very different.

Also, note that the Miller Formula was designed for long-range bullets with boattails. Flat-based bullets will typically stabilize in a slower twist rate, even if of the same length, and the 55 BK has a sort of compromise between a flat-base and a long boattail.

Don Miller admitted there were occasional problems in twist that couldn’t be predicted, and specific bullet shape is one. But even the best formula (and Don’s is considered one of the best, if not the best) can’t totally account for every nuance of bullet shape.

It also depends on entering correct information.

Well I did not see any place to change altitude, I did leave the temperature at default. If I put the temperature at -20 (Like in Montana)it gives a factor of 1.324 in the green. But very few shoot at the temperature. If it is change to 100 degrees it gets worse. It still will not get 55 gr Blitz King in .223 out of the red, let alone a 221 or Hornet. Just how many are using 55 gr V-Max and Blitz? I would think that Miller should state that it is not for varmint bullets. All the other calculators are very near the same. All showing 55 gr bullets are very marginal in .223.

The Miler Formula is available both on the Berger and Applied Ballistics websites, and the altitude setting is right next to the temperature setting.

Very cold weather like -20 makes a bullet less stable, not more stable. Warmer weather makes them more stable. Apparently you are doing something screwy.

In my experience the Miller Formula works pretty well for .224 varmint bullets, but not so much for .204's.

I have used Berger site and I see that it includes elevation. But I was using JBM – Calculations, it states it is Miller. After looking on internet I see that Miller has a calculator just for tip bullets. The Berger is not it, and is not correct for tip bullets. If you use JBM it does not include elevation, but if you enter the tip length (.15 on 50 or 55 gr Blitz Kings) you can see that is stable. I think Burger does not use it because they do not supply tip bullets. I can now see why 50 and 55 grain Blitz Kings work well with 1:12 twist. Now you might use the Miller from JBM for .204 it you use tip bullets.

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

http://arxiv.org/ftp/arxiv/papers/1410/1410.5340.pdf

http://arxiv.org/ftp/arxiv/papers/1410/1410.5737.pdf

Thanks John

McGoo,

JBM does include elevation--but it's stated as "pressure." While atmospheric pressure can vary some at the same elevation, elevation is by far the major factor in pressure. It's easier in most field siutations to use elevation rather than the exact barometric pressure. But you can come up with a close-enough barometric pressure from the elevation: Subtract 1 inch of barometric pressure for each 1000 feet above the 29.92 sea level standard. Thus 5000 feet above sea level would be about 25 inches of barometric pressure.

Aw, just get a 1-10 and be done with it....

Outdated theory. Stability over 1.5 is better. Use the Berger calculator as suggested above.

http://www.bergerbullets.com/twist-rate-calculator/



Or just buy a 1 in 10.

What a bunch of nerds.




Travis