bullet Length/Diameter - Twist rate...

[Anonymous]

The Greenhill formula is old and simple but still very reliable.

In reply to your question or statement concerning optimum velocity resulting in optimum accuracy.......IMO the answer is no. There may well be an optimum velocity for a particular rifle,load,bullet etc that achieves optimum accuracy but that result is derived from many factors.

What would be those many factors then? Granted a load may shoot different at 25 degrees versus 80 degrees, and at an altitude of 1000 ' versus 10,000', but are we talking thousands or inches in differences? At some point a load that works well I would think it's going to boil down to the bullet and powder charge driving it at a particular velocity.

Whether the formula is applicable or not - my 260 gr Dead Center is .429" in diameter and .9805" long which equates to a twist of 28.16. Both of my rifles are 1:28" so that works but I would think each would shoot this bullet/sabot differently at 1700 fps versus 2100 fps. Other than more or less powder which changes velocity, I'm not sure what else would be different if my loading practices were consistant. Any help would be appreciated.

"What would be those many factors then?"

Generally or essentially those common factors that determine accuracy . Good components consistently loaded that are "matched" to what your particular rifle likes. Harmonics have a good deal to do with accuracy. A particular load that shoots great in my rifle may not shoot worth a flip in yours. Does that make it a good load or a bad load? Obviously good for me but not for you. How components or loads behave in a particular rifle can only be determined by shooting.

I have ML rifles that will produce good accuracy with the same load/sabot/bullet over wide velocity ranges such as you mentioned. . ( I define good accuracy for a hunting rifle/load as one minute of angle. Consistent one inch groups at 100 yards out to 200 yards for ML rifles. If one is a "benchrest shooter" one inch groups at one hundred yards would suck! )

Twist rate does not determine if a barrel will shoot well or not, the quality of the barrel determines it accuracy potential. Yes we do need to use a twist rate that is generally matched to the projectiles we shoot but the "perfect twist" would not assure accuracy.

 

[tpcollins]

I found what I was looking for today! On another forum a guy sent me a stabilization chart where I can enter my bullet's data, twist, temp, bp, and by entering various velocities, I get see what's needed to get to a minimum SG of 1.4 - which was 2100 FPS. I think about 115 to 120 grains of BH209 ought to get close to that out of my BC. We'll see.