Thread: Fast twist barrels

Originally Posted by Mooseman

Question , Does a faster twist barrel ( 1-8) push bullets at a higher velocity than a 1-12 twist barrel. One rifle a 16 inch XBolt 223 the other a Rem Mod 7 20 inch barrel 223, both rifles using exact same load?
Haven't had a lot of experience with faster twist barrels so interested to see what people think.

It could lead to more pressure for the same load same as a bullet seated deeper in the same rifle should up pressure and therefor velocity but if you went and loaded both rifles the same pressure the 20 inch barrel should be faster (obviously some barrels are faster barrels but that's not due to twist rate. Personally I've never found such a things as too fast a twist but I think you can run into issues at really high speeds with light constructed bullets exploding mid air. A fast twist does give more versatility though in projectile selection.

The principal factors affecting velocity variations with an identical load in different rifles are barrel length, bore diameter and chamber dimensions. Twist rate is not a factor but is required for stabilisation of various bullet weights. The standard .223 twist is 1-12" which is designed for the 55 grain bullet, 1-9" for 62 grain, 1-8" for 68-69 grain, 1-7" for 69-77 grain. You will find an overlap of performance in practice but those are the optimum rates for gyroscopic stability. I was getting sub 0.5 MOA performance from my 1-7" Ranger AR with 69 grain BTHP Sierras until Jacinda got it. My replacement 1-8" Tikka CTR gives 0.6 MOA with the same load - still good enough for 350 metre rabbits!

Originally Posted by Stocky

Not quite that assumes that all energy from burnt powder is instantly passed on to the projectile but realistically it has intertia and take time to accelerate similar to dropping a bullet from a high building. It doesn't instantly reach terminal velocity. Just because all the powder is burnt doesn't mean acceleration stops. The bullet will still accelerate until the force from the pressure of the xplosion is equal and opposite to the drag from the barrel and air. If you want to see this go run a few loads though quickload etc. Complete burn is not the same as max velocity. Ie why you will see 7mm08 and 6.5 creedmoors with 30 inch barrels etc for target.

Spot on. The pressure curve keeps driving the bullet faster until it runs out of puff. Area under the pressure curve over time vs resistance. Combustion doesn't drop to zero the moment combustion finishes.

Here's a QuickLOAD analysis of a 223 loaded with a 40 gr V-Max at ADI's max recommended load. I just plucked this from ADI, it's not my pet 223 load, although its close to it.



The graph on the bottom left shows the pressure curve in red and the velocity in blue. The Chamber Pressure curve peaks after 1.28"/0.3 ms bullet travel and begins to fall but the Velocity continues to accelerate until the projectile exits the barrel.
Also of interest is the Amount Of Propellant Burnt is 91.3% indicating some powder being burnt after the projectile exits the barrel ie some wasted powder.

If running the same analysis with a 22" barrel instead of 24" the Chamber Pressure remains the same, Muzzle Velocity drops to 3435 fps, Amount Of Propellant Burnt inside barrel 90.46%, Barrel Time drops from ~0.95 ms to ~0.9 ms.

Originally Posted by zimmer

Here’s article from Accurate Shooter on the topic Twist versus Velocities.

How Muzzle Velocity Changes with Different Barrel Twist Rates « Daily Bulletin

And here’s Frank Green's (co-owner Bartlein Barrels) comment that he posted on the Oz F Class forum re that article.

“Not only did we make all those barrels in the test but we chambered them as well. So same chamber reamer was used in all of them and the barrels where all made at the same time. If I recall correctly all of the barrels where made out of the same lot of material except for the left hand twist was out of a new lot. So about as identical as you can make them.

It's always been my understanding that twist might have a small impact on velocity but for the most part it really doesn't in the grand scheme of things.

Later, Frank
Bartlein Barrels”

Greetings Zimmer and All,
There we have it. Twist makes no practical difference to velocity. Hard data should triumph over theory every time, especially theory not based on fact.
I saw an ad the other day for a Winchester rifle in .223. It had a 1 in 11" twist and was touted as being compatible with 55 grain projectiles. This is pure pandering to the ignorant. Both my 1 in 12" and 1 in 8" twist .223 rifles shoot the 55 grain projectiles equally as well and I expect that most will have the same experience. Perhaps the fast twist may shred some really tender projectiles but the thing that the 1 in 11" twist won't do is stabilise most of the high BC projectiles. Rant ended. Curmudgeon back in box.
Regards Grandpamac.

Originally Posted by gundoc

The principal factors affecting velocity variations with an identical load in different rifles are barrel length, bore diameter and chamber dimensions. Twist rate is not a factor but is required for stabilisation of various bullet weights. The standard .223 twist is 1-12" which is designed for the 55 grain bullet, 1-9" for 62 grain, 1-8" for 68-69 grain, 1-7" for 69-77 grain. You will find an overlap of performance in practice but those are the optimum rates for gyroscopic stability. I was getting sub 0.5 MOA performance from my 1-7" Ranger AR with 69 grain BTHP Sierras until Jacinda got it. My replacement 1-8" Tikka CTR gives 0.6 MOA with the same load - still good enough for 350 metre rabbits!

And some of these longer more modern projectiles are tending to show that extra twist helps with BC as shown by Byran Litz Testing of the Accubond long range projectiles.

No two 'identical' barrels produce the same velocity either, further skewing any results. Or rather it could be almost impossible to make identical barrels. I reckon this question could only be answered by a manufacturer using statistics from thousands of testfires...

Something i (if i were them) would never release. The moment it became known that there was a 50ft per second advantage having 1/7 twist on any model, the 1/8 would be chopped liver and hard to sell

Confusing the term "fast or slow twist" as also being directly related/connected to speed.

Originally Posted by T.FOYE

No two 'identical' barrels produce the same velocity either, further skewing any results. Or rather it could be almost impossible to make identical barrels. I reckon this question could only be answered by a manufacturer using statistics from thousands of testfires...

Something i (if i were them) would never release. The moment it became known that there was a 50ft per second advantage having 1/7 twist on any model, the 1/8 would be chopped liver and hard to sell

Yes never identical but they can get damn close. Matches guys often by multiple barrels and own there own reamer and can reuse load data from previous iterations to damn near the same results saving redoing load development. So if Bartlien does a test and limits variation even more by using a reamer sequentially limiting difference and pulling barrels from the same batch of steel and cutting them sequentially I'd say they are a pretty good authority to say that it's insignificant.
I definitely think we can conclude that the difference in the to rifles mentioned in the original post is not significantly impacted by the twist rates and is more likely a result of different batchs of powder, bore diameter, tighter chamber, faulty chromo or incorrect validation of speed.

Twist has nothing to do with velocity it is purely to stabilise the projectile, decide which projectile you want to shoot and the twist that will stabilise it
in a .223 up to 60gr 12" 69gr 9" 70-80 gr 8' 90gr 7". I had two 6mmBR's Barnard actions 30" True flite barrels chambered with the same reamer same load
one was 70fps slower than the other
It is the length of the projectile that determines the twist not the weight, The rotational speed of a fast twist can rip a light weight varmint bullet to pieces even in a .223.

Originally Posted by shooternz

Twist has nothing to do with velocity it is purely to stabilise the projectile, decide which projectile you want to shoot and the twist that will stabilise it
in a .223 up to 60gr 12" 69gr 9" 70-80 gr 8' 90gr 7". I had two 6mmBR's Barnard actions 30" True flite barrels chambered with the same reamer same load
one was 70fps slower than the other
It is the length of the projectile that determines the twist not the weight, The rotational speed of a fast twist can rip a light weight varmint bullet to pieces even in a .223.

All valid points except I wouldn't put true flite in the same repeatability league as Bartlien being buttoned vs cut and unless the barrels were made sequentially the more wear on the rifling button could partially explain any variation as could minute differences in chamber depth.

But regardless the point is moot in regards to question asked as twist rate doesn't effect velocity in any significant manner.

Does it effect wear and total life of the barrel? I know that's not the OP of the thread but i'm looking at replacing my barrel which is 1/8 with a 1/7.5 or 7 to be able to stabilise the longer projectiles

Originally Posted by T.FOYE

Does it effect wear and total life of the barrel? I know that's not the OP of the thread but i'm looking at replacing my barrel which is 1/8 with a 1/7.5 or 7 to be able to stabilise the longer projectiles

I don't think anyone could definitively say for sure. But I doubt it would be much if it did

There's a book by Bryan Litz that covers this (and other things) called "Modern advancements in long range shooting"
Apparently yes there is 5ft/sec in it. In both theoretical and practical testing.

I guess that it now turns into a question about which brands of projectile can handle being sent at 2800fts+ without flying apart...