LoadBase? Miller or Greenhill Method?

[Mike6158]

I'm wondering about something in the Twist and Stability section of the Analyzer tab in LoadBase. When I check the stability of my pet .308 round, the Miller method says under stabilized and the Greenhill method says fully stabilized. Since the particular round that I am checking shoots 1.4-ish groups at 300 yards my guess is that the Greenhill method is the one to trust. However... I haven't guessed right yet when it comes to ballistics... so I figured that I would ask.

 

[BryanLitz]

Unless your bullet is a round nose, flat base bullet with a muzzle velocity of ~1400 fps or less, the Greenhill formula is not the one to use.

The Miller formula is tailored for modern bullets at high velocities and will give much better stability predictions than Greenhill.

Having said that, if all your inputs are right and it suggests you're 'understabilized' for the bullets you shoot great groups with, the following are things to think about:

1) The formula isn't 100% perfect.
2) The thresh-hold for a 'safe' margin of stability is subjective. Some say a stability factor of 1.5 is required, some say 1.3. I go with 1.4.
3) Your barrel may have a faster twist than you think.
4) Are you shooting at high altitude, and is this reflected in the stability calculation you did?

Just some thoughts that might rectify the calculations and observations.

-Bryan

 

[Mike6158]

Unless your bullet is a round nose, flat base bullet with a muzzle velocity of ~1400 fps or less, the Greenhill formula is not the one to use.

The Miller formula is tailored for modern bullets at high velocities and will give much better stability predictions than Greenhill.

Having said that, if all your inputs are right and it suggests you're 'understabilized' for the bullets you shoot great groups with, the following are things to think about:

1) The formula isn't 100% perfect.
2) The thresh-hold for a 'safe' margin of stability is subjective. Some say a stability factor of 1.5 is required, some say 1.3. I go with 1.4.
3) Your barrel may have a faster twist than you think.
4) Are you shooting at high altitude, and is this reflected in the stability calculation you did?

Just some thoughts that might rectify the calculations and observations.

-Bryan

Have you ever heard the term "garbage in - garbage out"? I can't believe what I did. I'll bet I checked the input 10 times and each time I thought "I need to go measure the length of the bullet. 1.3" seems a tad long."

The bullet length was wrong in the program. The 150gr Sierra Pro-Hunter (.308) (at least the one that I mic'd) is 1.07" not 1.3" which is apparently the program default. Both Miller and Greenhill say over stabilized now (2.26 and 2.46 respectively).

Now that that's over I can play with the 168gr Berber VLD numbers

Thanks Bryan