Fast Twist --- Light Bullets

[bsekf]

My research shows that light bullets in a fast twist are rotating so fast that they lose accuracy. What happens if you reduce the powder charge to lower the velocity? I am going to build another 6 or 6.5 x 47L and would like to shoot some lighter bullets along with the 105's and 130's. Anybody with some "real world" experience with 68-70 gr 6mm bullets in a 1 in 8, or 90 gr 6.5mm bullets in a 1 in 8?

Bill

 

[Sniper338]

Theres no one barrel twist that fits every bullet, thats why different twist rates are available... you either pick one or the other to match the bullet of choice, or you get multiple barrels spun up...

 

[Fotheringill]

I do not shoot your caliber, but when I started shooting my 1:7 twist Savage 12, 55 grain rounds weren't even on the board, no less the 10x12" paper target at 100 yds. I was told the twist was too fast for that round. I changed to 73 grain bullets and no problems. 80's were better.

 

[snoopy001]

I do not shoot your caliber, but when I started shooting my 1:7 twist Savage 12, 55 grain rounds weren't even on the board, no less the 10x12" paper target at 100 yds. I was told the twist was too fast for that round. I changed to 73 grain bullets and no problems. 80's were better.

If you have a look on the ballistic calculator the bullet length is much more twist sensitive as the given b.weight.

 

[Hotshot]

A lot of shooters on the Saubier small caliber forum report excellent results with light bullets in fast twist barrels. We're mostly dealing with 17-22 caliber over there but 6mm shooters are not kicked off the forum. Some of the thin skinned bullets will spin themselves apart at too high of a speed so we slow them down. The fast rotation sure raises heck with a varmint. No hard and fast rules seem to exist on over stabilizing a bullet if there is such a thing. Some think once a bullet is stabilized you can give it as much more spin and velocity as it will take until flying apart. Try it and let us know, we're all learning here.

 

[liseo]

With 6 BR 1/8 twist and Barts/Gentner bullets, I can make 1/3 MOA 5 shots groups at 100 meters.

Even I was surprised by the result. But was using custom barrel. Maybe a more rough barrel did not have this result

 

[rayporter]

6mm 55 gr Bal tips and an 8 tw work for several rifle I have.

 

[KMart]

If you can live with 80 grain Bergers being light, I have an 8 twist 6 Dasher that will shoot them at 3380fps.
Last Thursday, I shot a 5 shot group that measured .657 at 400 yards. The 80 Berger is long and will jam in a 104 freebore and have plenty left in the neck.

 

[T 700]

I have a 6br 60ths free bore with 8 twist barrel and shoot 70gr blitzkings to 105HVLD both with great accuracy, you will just have to see what your barrel likes.

 

[Ned Ludd]

In my hands, it is far better to overspin a bullet than to underspin it. I've fired bullets as light as 40 gr loaded in commercial ammunition from a precision bolt rifle with a 7-twist .224 barrel and they shot just fine (~ half MOA), considering they were factory loads. I've fired 155 gr Lapua Scenars from a bolt rifle with an 8-twist .308 barrel. They also shot just fine. Grossly overspinning a bullet is not ideal, but it's better to purchase a barrel and have it chambered for the heaviest (longest) bullet you intend to shoot, rather than the other way around. You can still shoot lighter (shorter) bullets out of the faster twist barrel reasonably well, even if you're not getting every last little scrap of precision that you might with the optimal twist rate for that particular bullet. If the precision is absolutely horrible with the lighter bullet in a fast twist barrel, chances are good it wasn't solely because of the twist rate.

 

[boltfluter]

I have shot many groups of .375 or smaller from my 8 twist .22BR with the 40 gr V-max bullets. Coasting along at 3750 fps.

Paul

 

[dcali]

You are correct in that overspinning a bullet will degrade accuracy. Roughly in proportion to the increase in twist rate.

The best way to address this is to use a slower twist barrel and give up the ability to stabilize heavier bullets.

If you don’t want to do that, I don’t think you will have much luck playing with velocity outside of the normal load development process. To be more direct, the exact impact of velocity will be bullet dependent. But don’t expect dramatic changes.

You will still get the accuracy benefit of shorter bullets even with a faster twist, but they won’t be as accurate as they could be woth a slower twist.

Basically, what you want is going to be a compromise. Any specific bullet has an optimum twist for a specific application. For long range shooting you want a bit of margin to wring ever bit of BC out of the bullet, and short range you basically want the bullet on the lower edge of stability to maximize precision. This is why short range benchrest guys use slow twists and stubby bullets while long range guys spin up heavy bullets pretty quickly.

Most shooters tend to have a healthy margin on their twist rate. I would say that they tend to overdo it in fact. But it’s better to know the bullets will be stable than to wring out that last 5% of accuracy in most applications.

 

[DirtySteve]

The light bullets loose accuracy due to slight bullet imbalance. Over spinning exaggerates the effect. Premium bullets like Bergers/JLK...ect. Are less likely to be unbalanced and should fly straight. In fact this seems like a good way for hard core bench resters to test bullets for balance and then shoot in a slower twist. Just a thought...

Also the smaller the bullet diameter the harder it is to make a balanced bullet. Some companies are better at it than others.

 

[HTSmith]

If you can live with 80 grain Bergers being light, I have an 8 twist 6 Dasher that will shoot them at 3380fps.
Last Thursday, I shot a 5 shot group that measured .657 at 400 yards. The 80 Berger is long and will jam in a 104 freebore and have plenty left in the neck.

The 80 Gr Bergers work well in my 6BR 8 twist also. I got best accuracy when I pushed them fast.

 

[bsekf]

Doesn't a 100 grain bullet in a 1in8 spin at the same RPMs as a 130 grain bullet in a 1in8 if they are both at the same speed?

 

[dcali]

Doesn't a 100 grain bullet in a 1in8 spin at the same RPMs as a 130 grain bullet in a 1in8 if they are both at the same speed?

Yes

 

[K22]

I recently purchased a Tikka 223 with an 8" twist. I've shot 223's for many years, all with 12" twists and 50 / 55 grain bullets so this was my first experience with the 8" significantly faster twist.

When I tried the bullet / load combo I've used for years, i.e. 55 Nosler BT's, I had trouble getting groups smaller than 1 1/2" with this new rifle. I began testing by reducing the powder charge, to slow things down, and the groups finally tightened to about .75" or half.

Next I tried the 60 Vmax with various powder charges. I found the same thing again, the lighter charges shot significantly better so I think there may be something to you're assertion. In addition, the 60 Vmax', i.e heavier bullets, were yielding better groups also, sub 1/2".

 

[mike a]

Doesn't a 100 grain bullet in a 1in8 spin at the same RPMs as a 130 grain bullet in a 1in8 if they are both at the same speed?

Wow that's a teachable moment for me. That's easy to understand and sure makes sense. Thanx

 

[Homerange]

I recently purchased a Tikka 223 with an 8" twist. I've shot 223's for many years, all with 12" twists and 50 / 55 grain bullets so this was my first experience with the 8" significantly faster twist.

When I tried the bullet / load combo I've used for years, i.e. 55 Nosler BT's, I had trouble getting groups smaller than 1 1/2" with this new rifle. I began testing by reducing the powder charge, to slow things down, and the groups finally tightened to about .75" or half.

Next I tried the 60 Vmax with various powder charges. I found the same thing again, the lighter charges shot significantly better so I think there may be something to you're assertion. In addition, the 60 Vmax', i.e heavier bullets, were yielding better groups also, sub 1/2".

69 gr Bergers is what the guys here use in 8" twist Tikka's.
2" groups out @ 500 !

 

[bsekf]

mike a, here is the rest of the story. Wouldn't it be harder to make a "balanced" 130 gr. bullet than a balanced 100 gr bullet????? Therefore, if the 100 gr. bullet doesn't vaporize, why isn't it as accurate as the 130? There would be a change in barrel harmonics and that might be the factor.

But, after all this theroy, I believe light bullets in a fast twist shoot better at lower velocity.

Bill