Bullet Weight/Twist Rate Charts versus accuracy

[Oly]

The likes of Lilja offer charts showing which bullet weights are most appropriate for the twist rate of your barrel. But presumably bullet weights in the middle of this range are more accurate than bullet weights at the extremities? If so how much more accurate? 

For example a 308 with 1 in 10 twist shows as being able to use bullet weights from 168gn to 200gn...so does that mean that a weight in the middle (circa 185gn) will be more accurate than those at the top (200gn) or bottom (168gn) of the range? 

If so is this likely to be a measurable difference in grouping size? 

https://riflebarrels.com/calibers-and-twist-rates/

[Popsbengo]

I don't think it's a simple answer.  Muzzle velocity also contributes to spin rate and bullet weight isn't a simple concept either as length and axial distribution matter too.  Add in the drag effects and bullet profile etc etc...

Obviously calculated effects are important to get into the ball-park but I'm a firm believer in empirical testing - shoot it and see 😁

[KABOOM]

The chart is nice it recommend twist rates and wonderful for that, what will group better or worse requires shooting and results on target

.when bullets were not as well balanced/made as they are now maybe this chart would have been more able to provide a clue, honestly I prefer to shoot and see my results.

 

[Brillo]

This is an interesting question. 

Berger provide a twist rate calculator which gives optimum rates for different bullets weights. Most bullet manufacturers give optimum twist rates for their bullets but this is about stability, not necessarily accuracy. You would think the two go hand in hand but we all know that’s not necessarily true.

Many top shooters believe the slower you spin a bullet within the recommended twist rate for a bullet weight, the more accurate it will be, especially at high MVs. For example, many top FTR shooters use a 1:14 twist barrel, which at 3000 fps has an SG of only 1.17 and is just marginally stable, but these guys consistently achieve groups of less than 0.25 MOA.

The other consideration using a faster than recommended twist rate. I was told by a well respected gunsmith that if you spin a bullet to fast the phenomenon of precession creeps in. This would obviously affect accuracy but how would be seen in terms of, say group shape?

[Richiew]

After reading the Brian litz books gyroscopic stability seems to be a big factor at transonic speeds so a high sg Above 1.5 means very stable . high bc doesn’t always go with stability so you need to choose a bullet that fits your barrel twist and speed  . For a 1/12 “ twist lapua 155s don’t do too good but older shaped bullets such as 175/155 MatchKings do very well . So it’s a balance of bc and stability and speed along with suitable twist rate . The lapua s work great with one in ten or eleven twist so not so good in my accuracy international 308 but good in a mates Sako trg . Hybrids or secant shaped bullets look like they are happier with faster twists and the tangent type like a slower twist  . Have look at the books Ballistic performance of rifle bullets you’ll be amazed at how bad and good some bullets are and be able to pick bullets that might fit your rifle without too much heartache . Cheers Rich 

[KABOOM]

19 hours ago, Brillo said:

This is an interesting question. 

Berger provide a twist rate calculator which gives optimum rates for different bullets weights. Most bullet manufacturers give optimum twist rates for their bullets but this is about stability, not necessarily accuracy. You would think the two go hand in hand but we all know that’s not necessarily true.

Many top shooters believe the slower you spin a bullet within the recommended twist rate for a bullet weight, the more accurate it will be, especially at high MVs. For example, many top FTR shooters use a 1:14 twist barrel, which at 3000 fps has an SG of only 1.17 and is just marginally stable, but these guys consistently achieve groups of less than 0.25 MOA.

The other consideration using a faster than recommended twist rate. I was told by a well respected gunsmith that if you spin a bullet to fast the phenomenon of precession creeps in. This would obviously affect accuracy but how would be seen in terms of, say group shape?

A very interesting concept, perhaps im more simplistic, I don't compete just long range varminter and I mostly wont shoot much past 500 yds. My theory is slightly faster twist than required for a given bullet, could well be wrong but its working for me.

 

[VarmLR]

20 hours ago, Richiew said:

After reading the Brian litz books gyroscopic stability seems to be a big factor at transonic speeds so a high sg Above 1.5 means very stable . high bc doesn’t always go with stability so you need to choose a bullet that fits your barrel twist and speed  . For a 1/12 “ twist lapua 155s don’t do too good but older shaped bullets such as 175/155 MatchKings do very well . So it’s a balance of bc and stability and speed along with suitable twist rate . The lapua s work great with one in ten or eleven twist so not so good in my accuracy international 308 but good in a mates Sako trg . Hybrids or secant shaped bullets look like they are happier with faster twists and the tangent type like a slower twist  . Have look at the books Ballistic performance of rifle bullets you’ll be amazed at how bad and good some bullets are and be able to pick bullets that might fit your rifle without too much heartache . Cheers Rich 

This.

 

It's not so much choosing a bullet in the middle of the accepted wisdom on weight limits for twist but on the best form factor for weight to achieve the most stable outcome.  Brian Litz's Applied Ballistics charts are your friend...

[Oly]

Lots of great wisdom imparted there gents. Many thanks 👍