JBM ballistics calculator question?

[Free Trapper]

Hi all, newbie question looking at bullets stability using JBM Ballistics calculators. Output Data. I understand the three colors red orange and green, green being good red poor. But what those number (1.3-2.0) mean? Playing around doing different numbers came up with 4.5 in the green. Somewhere I read stability value should be in the range of 1.3 to 2.0 to ensure bullet stability. Going to post but I'll keep on reading, thanks. Free Trapper

JBM - Calculations - Stability

jbmballistics.com

 

[FeMan]

The stability number is dimensionless as far as I know. A stability factor of 1.5 or higher is generally considered to be stable in flight. As the number drops below 1.5, you start to flirt with potential instability. Partially stable bullets (SF in the 1.3 to 1.4 range) may still fly straight and group well, but could start to have a yaw in their rotation which will result in a reduced BC coefficient. Once they become unstable, they will scatter and go through the paper (if you hit it) sideways.

Be sure to input everything correctly. The tab you provided has the bullet length of a 30-cal 220 grain at 1.5 inches, which is probably too short and results in a higher calculated SF. Stability is highly affected by bullet length as well as altitude and twist rate so make sure you enter that correctly so you have dependable results.

 

[xr650rRider]

Miller stability formula

another article by Don Miller

 

[chromatica]

If a bullet wasn't spinning, it would flip over and fly backwards down the range. That is because the center of pressure is in front of the center of gravity. It is only the angular momentum imparted by the rifling of the barrel that resists the overturning force (actually overturning moment). The Miller stability index is a ratio of the angular momentum to the overturning force. The OP is correct in his statement that you want the Miller Stability Index to stay above the danger zone of 1.2 to 1.3 to insure stable flight.

 

[Ranger188]

Here's a different one.
Make sure you fill in all 8 spots that relate to your bullet and conditions.
If it's a Berger bullet, use the pull down.

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.

bergerbullets.com

 

[jelenko]

That one doesn't calculate stability, does it?

 

[Jeff Porter]

The twist rate STABILITY calulator?

 

[Jeff Porter]

Air temperature is a big mover in that calculator. Doesnt take much of a drop in Temp to make a bullet mariginal.

 

[Ned Ludd]

Gyroscopic stability coefficient (Sg). Ideally, 1.50+ will net the full intrinsic BC of a given bullet. However, even an Sg as low as 1.30+ will usually generate sufficient gyroscopic stability that the bullet doesn't "fall out of the sky, but it may behave as though its BC is lower than advertised. You can find a more detailed discussion of Sg and its effect on BC: https://www.amazon.com/Applied-Ballistics-Long-Range-Shooting/dp/0990920615

 

[jelenko]

The twist rate STABILITY calulator?

The link has been changed. It orignally was a link to the JBM trajectory calculator.

 

[XTR]

As noted in Ned Ludd’s post above, a lowish Sg doesn’t mean your bullet is going to the target tumbling, and 10 yrs ago before Bryan Litz started publishing it was commonly accepted that a 1.4 was plenty. In articles and books Bryan postulates that with an Sg below 1.5 a bullet loses some % of its inherent BC due to “wobble” ; i.e., not constantly presenting its best nose forward profile to the air.

That said, unless you are shooting past 600 yds losing 5% of the BC isn’t a big deal. It’s not making your bullets less precise. They will still go where you point them. You just get an extra half a bullet diameter of wind drift

 

[Free Trapper]

The stability number is dimensionless as far as I know. A stability factor of 1.5 or higher is generally considered to be stable in flight. As the number drops below 1.5, you start to flirt with potential instability. Partially stable bullets (SF in the 1.3 to 1.4 range) may still fly straight and group well, but could start to have a yaw in their rotation which will result in a reduced BC coefficient. Once they become unstable, they will scatter and go through the paper (if you hit it) sideways.

Be sure to input everything correctly. The tab you provided has the bullet length of a 30-cal 220 grain at 1.5 inches, which is probably too short and results in a higher calculated SF. Stability is highly affected by bullet length as well as altitude and twist rate so make sure you enter that correctly so you have dependable results.

FeMan
Be sure to input everything correctly. The tab you provided has the bullet length of a 30-cal 220 grain at 1.5 inches, which is probably too short and results in a higher calculated SF. Stability is highly affected by bullet length as well as altitude and twist rate so make sure you enter that correctly so you have dependable results.

Thanks FeMan, you answer my question and not knowing provided link-showing calculator I was using. Input was random numbers trying to understand output data. I have 450 RN and 500 RN-grain bullet (.492 dia) from Buffalo Arms for paper patching. 1.0560 length 450-grain RN and 1.1415 length for 500-grain RN, muzzle velocity guessing around 1350.0 ft/s . GM barrel 1:28. I did not see input for altitude but its 1,650 ft when calculated output data stability shows 2.211 and 2.062 for those two bullets. Also have a cast lube bullet 500 S&W Magnum (.501dia) .8985 length 440-grain FN, output data stability 3.485. I was hoping this bullet would work for me but at 50 yards wide spread and some keyhole on target. The two round nose paper patch bullets from buffalo Arms group around 3 inch at 50 yards using iron sights. Thinking had I sighted in at 6 o'clock on 3 inch bulls would have done better as front sight completely cover 3 inch bulls at 50 yards. When doing some more reading saw my paper patch was wrap to long past the ogive on bullets. Shorten wrap but have not shot them yet will be looking for improvement. I want to build my own swaging dies; modified an old pistol die trying to copy that 500 S&W Magnum for paper patching. It was far from perfect lol but learn a lot in doing so, thinking the need to copy those round nose from Buffalo Arms as they are not cheap. Will post pictures of my first attempt at swaging if I can figure how too, thanks. Free Trapper