1x7 or 1x9 twist rate ...QUESTION

[HAMMERDROP]

If a 1x7 twist rate in a 14.5" barrel stablizes heavier bullets better than a 1x9 in that length barrel - WTF happens to a 55gr bullet or lighter when fired thru a 1x7 in 14.5 inch length ?
Is there a loss of velocity?
Does accuracy suffer greatly?
Does it break glass way off to the left somwhere?
Or does bullet weight and twist rate only come into play when placing a long distance shot 200-300m ??

Thanks ,
Michael

[Alpha Sierra]

Light bullets in fast twists: OK

Heavy bullets in slow twists: Not OK.

[HAMMERDROP]

1x9 is considered a 'slow' twist rate and 1x7 is considered 'fast' ...thanks

[jmart]

75 grain stuff in 1 in 9" is at the margin -- it's a hit or miss proposition. The longer the barrel length the better off you are since you'll generate higher rpm's with the faster starting velocity.

With 1 in 7" you are firmly grounded with heavyweights.

Anything lighter than 75 grain will run fine in 1 in 9".

55 grain ball runs fine in 1 in 7" also. Not sure though how light you can go though before accuracy and/or bullet integrity issues crop up. I'll defer to others to chime in.

[baffle Stack]

Hammerdrop, I have fired thousands of 55gr rounds thru my 14.5 1/7. This combo is good to go.

[KevinB]

I've shot 40gr bullets in a few 1:7 barrels.
It seems to depends on the load (pressure/velocity) and the bullet - as whether or not it will come apart in flight, or have poor accuracy due to damage to the bullet jacket. However some loads will shoot quite well.

Most of those loadings have very light/thin bullet jackets - and it does not seem to do well with the 1:7 twist

[Hawkeye]

Barrel length has no physical effect on accuracy or how fast a bullet turns. It only affects velocity and trajectory.
If a barrel has a 1/9 twist, a bullet fired through it will be making one turn every nine inches regardless of its length. Same for 1/7 or any other twist rate. In other words, it you take a 10" 1/9 twist barrel and a 20" 1/9 twist barrel, when the round exits both of those barrels it will be turning at the same rate. Once every 9".
As to the original question, firing a 55 gr bullet through a 1/7 twist barrel will just result in it turning once every 7" vs once every 9" through a 1/9. I havent seen any accuracy issues and cant think of any physical reason there would be.
The heavier the bullet, the faster it needs to spin to stabilize itself in flight. The lighter the bullet, the slower it can turn and still achieve stabilization.

Mind you, I am not a engineer or physics expert, and have been known to be incorrect on multiple occasions. However, I have stayed at a Holiday Inn once or twice, and even a Raddison.

[Milkman]

Barrel length has no physical effect on accuracy or how fast a bullet turns. It only affects velocity and trajectory.
This is a contradiction. When a projectile looses velocity and become unstable it most assuredly looses accuracy. The length of the barrel does in fact effect how fast the bullet spins.


If a barrel has a 1/9 twist, a bullet fired through it will be making one turn every nine inches regardless of its length. Same for 1/7 or any other twist rate. In other words, it you take a 10" 1/9 twist barrel and a 20" 1/9 twist barrel, when the round exits both of those barrels it will be turning at the same rate. Once every 9".
Again, this is not correct. The bullet may spin one complete revolution in 9 inches but in the 20" barrel the projectile is in fact spinning faster because it is traveling faster. For instance, if you fired a 77gr bullet out of a 10" 1/7 barrel it would still be spinning slower than a 77gr bullet fired from a 20" 1/9 barrel. If you look at the majority of the bolt action .223 rifles you will see most are 1/9 and slower but still fully capable of stabilizing 77gr and heavier projectiles.

[K.L. Davis]

How fast a bullet spins (rpm) is a function of the rate of twist (rot) and the velocity of the bullet... so yes, in most cases with two barrels with the same ROT, the longer barrel will spin the bullet faster (rpm) -- this is why you can use a slower ROT on longer barrels and still get good stabilization.

Plastic deformation failure, caused by spinning the bullet fast enough to actually tear it apart is not that common in ARs, it is largely dependant on the jacket strength of the bullet and seen mostly is very lightweight, thin jacket rounds.

The ROT is determinded by the length of the bullet really, but as longer bullet usually weigh more than shorter bullets, the weight is sort of the quick and easy way to keep track of it... but the long formula for figuring rate of twist for proper stabilization does not take into account the weight at all.

Faster ROT does reduce velocity, but very, bery slightly... it is simple physics thing, more energy is used to spin the bullet, therefore that energy is lost on making it go faster... again, the difference in FPS is single digits in most cases.

You can "over stabilize" a bullet really... I know there is supposed to be no such thing, but in stability factors of more than 1.5, the attitude of the bullet changes to a more nose up flight... it really does not effect accuracy in a way that 99.9% of people even care about -- no more than the increased RPM will cause more spin drift, all things that if you are shooting a 14.5 barreled AR, you really do not even need to know exist.

Bottom line... 55gr from 1:7 14.5" barrel = no problem, but a lot of pretty smart folks are going to 1:8 as the "universal" ROT. :)

[Alpha Sierra]

a lot of pretty smart folks are going to 1:8 as the "universal" ROT. :)
Highpower shooters using ARs have pretty much settled on 1/8 as the standard since at least four or five years ago.

It will shoot everything from 45 to 80 grains with sub MOA (often in the fives) accuacy. More than good enough for any use short of benchrest competition.

[baffle Stack]

This is a contradiction. When a projectile looses velocity and become unstable it most assuredly looses accuracy. The length of the barrel does in fact effect how fast the bullet spins.

Not if you measure as Hawkeye did, Revolution per distance traveled. (Rev/unit length. In this case 9 or 7 inches) With this unit of measurement Revolution would not be dependent on barrel length. However if you measure revolutions per unit time (Rev/unit time) Then you are technically correct because longer barrel = more velocity (distance traveled / change in time) and in turn that = faster revolution because it is traveling the same unit of length only quicker. It gets confusing because we’re talking of both revs/length and revs/time



Again, this is not correct. The bullet may spin one complete revolution in 9 inches but in the 20" barrel the projectile is in fact spinning faster because it is traveling faster. For instance, if you fired a 77gr bullet out of a 10" 1/7 barrel it would still be spinning slower than a 77gr bullet fired from a 20" 1/9 barrel. If you look at the majority of the bolt action .223 rifles you will see most are 1/9 and slower but still fully capable of stabilizing 77gr and heavier projectiles.

Technically yes. Again just look at Hawkeye's unit of measurement and you will see he is not incorrect at all. He is using the unit of measure that all our barrels stamped with. Rev/length. i.e. 1/7, 1,9, which has nothing to do with time.

[K.L. Davis]

Highpower shooters using ARs have pretty much settled on 1/8 as the standard since at least four or five years ago.

It will shoot everything from 45 to 80 grains with sub MOA (often in the fives) accuacy. More than good enough for any use short of benchrest competition.

About 5 years ago, I had some barrels made up: Stainless, Wylde Chamber, 1:8, 16" mid-length gas system -- I tried to sell them :(

At the time, all of the *experts* found elsewhere where like...

"One in eight? what is that? And no one would want a stainless barrel, IT IT'S NOT CHROME, GO HOME!... that mid-length thing is a fad... what a waste of money"

I have been shooting 1:8 for a long time...

[K.L. Davis]

... some stuff was here ...

Not sure what you are asking... figuring rate of twist for stability is determined by actual RPM of the round more than anything.

In order to get this, we get the velocity over a reasonable distance and just assume that it is constant (of course it is not, but you will see how it works).

So, say we have a velocity of 2700FPS, and a ROT of 1:7

We want to make everything feet, so divide 7 by 12 and the ROT is one revolution every 0.583 feet.

The round travels 2700 feet in a second, so by dividing 2700 by 0.583, we can see how many times it will spin in one second... 4631 RPS

Then we multiply that by 60 to get the revolutions per minute... 277,873 RPM

[Alpha Sierra]

About 5 years ago, I had some barrels made up: Stainless, Wylde Chamber, 1:8, 16" mid-length gas system -- I tried to sell them :(

At the time, all of the *experts* found elsewhere where like...

"One in eight? what is that? And no one would want a stainless barrel, IT IT'S NOT CHROME, GO HOME!... that mid-length thing is a fad... what a waste of money"

I have been shooting 1:8 for a long time...
Ain't it sad what the herd mentality and lack of critical thinking does?

[Milkman]

Not if you measure as Hawkeye did, Revolution per distance traveled. (Rev/unit length. In this case 9 or 7 inches) With this unit of measurement Revolution would not be dependent on barrel length. However if you measure revolutions per unit time (Rev/unit time) Then you are technically correct because longer barrel = more velocity (distance traveled / change in time) and in turn that = faster revolution because it is traveling the same unit of length only quicker. It gets confusing because we’re talking of both revs/length and revs/time



Technically yes. Again just look at Hawkeye's unit of measurement and you will see he is not incorrect at all. Now can someone find the relation between revs/length and revs/time? That would be interesting. I have an idea but I don't want to bore you guys nor hijack the tread :p

I read exactly what was typed and answered accordingly. A projectile leaving a 10" 1/9 barrel is not turning at the same rate as a projectile leaving a 20" 1/9 barrel. Barrel length does have any effect on accuracy and is a condition for reaching stabilization of a projectile. No where was "revolution per distance" mentioned, only "rate."

[baffle Stack]

I read exactly what was typed and answered accordingly. A projectile leaving a 10" 1/9 barrel is not turning at the same rate as a projectile leaving a 20" 1/9 barrel. Barrel length does have any effect on accuracy and is a condition for reaching stabilization of a projectile. No where was "revolution per distance" mentioned, only "rate."

Would not "revs/distance" be a measure of rate? Meaning 1 rev per unit length measured. That seems like a rate of twist to me. Its just a funny one because it is not relative to time like we're are all used to.

I'd be happy to discuses this further Milkman, but lets do it via IM as not to spam Hammerdrop's thread.

[baffle Stack]

Not sure what you are asking... figuring rate of twist for stability is determined by actual RPM of the round more than anything.

In order to get this, we get the velocity over a reasonable distance and just assume that it is constant (of course it is not, but you will see how it works).

So, say we have a velocity of 2700FPS, and a ROT of 1:7

We want to make everything feet, so divide 7 by 12 and the ROT is one revolution every 0.583 feet.

The round travels 2700 feet in a second, so by dividing 2700 by 0.583, we can see how many times it will spin in one second... 4631 RPS

Then we multiply that by 60 to get the revolutions per minute... 277,873 RPM

Don't worry about it. I think I errored in my quoted text. But thanks for the info still.

[T-700wrench]

First Post
Nice forum
The only time I have had trouble with a 1/7 twist with any bullet was with a Pacnor 22-250. The 50 gr Vmax would blow up at around 3900. They were still incredibly accurate, except for the ones that made a puff of smoke about 50 yards downrange. I shoot them all the time in 20'' A2 barrels. They will blow up in a coyote and not leave an exit wound at as close as 60' or as far as 310 yards at a mv of 3300. The 80 grain sierra will blow in a 1/7 at 3850. This I learned from a 22/6mm Ackely. :D

[royta]

OK, so 55 FMJ in 1/7 is just fine. However, I didn't read anything about 55 Soft Point or Ballistic Tip. I'm guessing the standard jacket Soft Points will be just fine and very accurate, but I'm not sure what would happen to the thinner jacketed bullets like a V-Max. Anybody out there with experience with 1/7 16" barrels and 55 grain hunting/varminter bullets?


T-700wrench - I also have a Pac-Nor .22-250. It's a 1/12 though. Super Match Stainless Steel, 3-groove, 24", #5 Sporter contour. Awesome!