Barrel twist question for a short barrel 9mm pistol

[Rebel_Rider1969]

Seeing 3 types, 1:10, 1:9.85, 1:16.
Glock 9mm g26.
What's the best twist? For 115, 124 or lighter.

 

[Rebel_Rider1969]

?

 

[FLT]

1 in 10 is all I’ve ever had , it seems to be the standard as best I can tell.

 

[ccc]

Wow, never even cared when it comes to a pistol. In fact never even heard this come up when it involves a pistol. I would not think it mattered at all when it comes to a conceal carry pistol. I am curious to hear other opinions.

 

[Jevaughn]

With rifles, the longer projectiles require faster twist rates to stabilize than shorter projectiles (generally, the heavier the bullet the longer it is, but not always). With a short barreled pistol, I'm sure it plays a part for stabilization, but I've never honestly seen it come up. I'd imagine that you wouldn't see much difference between the 1:10 and 1:985 but might in the 1:16. I'd probably go the 1:10 route unless you like shooting the 80-90gr hyper velocity stuff. If you like those liberty munition 50gr m monolithic holdings that zoom at 2000+fps, that 1:16 barrel would probably give you great results but not do well with 124gr pills

 

[Rebel_Rider1969]

From more reading it appears target guns use the slower twist. Sw and colt revolvers use 1:12, 1:16 38 spc, 357m. Now who knows. Also KT PF9 uses 1:16. Very accurate.

 

[72sofsme]

So, here is a page that gives a ton of knowledge on the subject of twist rates to help partially answer the question asked ...

Q. How do you determine the rifling twist needed to stabilize a given bullet?
A. The needed rate of twist is affected by the diameter of the bullet, the bullets weight, and the bullet's overall length. Longer bullets need a faster twist to stabilize. As an example, a 1:12 twist in .30 caliber will adequately stabilized most commercial bullets of up to about 175 grains. To use a heavier (longer) bullet or to obtain optimum stability and accuracy with long pointed or boat tailed bullets of that weight requires a 1:10 twist. For best accuracy the slowest twist that will stabilize the bullet should be used.

Modern bullet stability calculations are based upon the work of the late Robert L. McCoy who was a ballistician with the Ballistic Research Laboratory at Aberdeen Proving Ground. His work, now used in advanced ballistics programs, accurately takes into account all of the factors involved in bullet stability and accurately describe the bullet's behavior at a given twist rate and generates a stability factor. A stability factor of from 1.25 to 2 is considered "stable."

There is an old formula called the Greenhill Formula that, while it was designed for estimating twists for boat tailed lead core bullets of moderate velocity, does a pretty good job of estimating twist required for flat based bullets under "normal" conditions.

T = Twist in inches
K = Greenhill's constant = 150 (This has to do with the specific gravity of a jacketed lead bullet)
D = Bullet diameter in inches
L = Bullet length in inches

T = (K * D2) / L

Using a 1.35 inch long .308 bullet (173 gr) and crunching the numbers we get about 10.5 (One turn in 10.5", which is pretty close to the 1:10 twist normally used in .30-06 rifles. The twist for the .308 is nominally 1:12 because it was based on the shorter bullet of the 150 gr military ammunition from which the .308 commercial round was developed. Most match rifles in .308 have a 1:10 twist to stabilize the 180 and 190 boat tailed match bullets better. The results from the Greenhill formula are on the conservative side--indicating a faster twist than probably needed. That doesn't cause any problems because a little too much stabilization is better than too little.

However, the one big catch with the Greenhill formula is that it does not account for the effects of temperature or muzzle velocity. As temperature or velocity decreases a faster twist is needed to maintain the same level of stability. Colder and thus denser air has a more destabilizing affect than warmer air. A lower muzzle velocity results in a slower rotational speed of the bullet and thus less stability.

As an example of the effect of temperature, the original M16 rifle for the M193 55 gr ball came out with a 1:14 twist which was barely stable at 68 degrees and which was totally unstable below about 40 degrees. They changed to a 1:12 twist to get stability (barely) at colder temperatures. The new M855 62 gr round is unstable below about 65 degrees with the 1:12 twist and requires a 1:9 twist to be stable. They went to a 1:7 twist because the M856 tracer round has a very long bullet, but the ball round does just fine in 1:9.

For velocity issues long bullets are more susceptible than short ones. For example the above 173 gr .308 bullet in a 1:12 twist barrel is stable from at muzzle velocities of 4000 f/s down to about 500 f/s . Below that muzzle velocity it becomes unstable. If we turn the design into a VLD bullet with a length of 1.45" the bullet is stable for muzzle velocities from 4000 f/s to 3,000 f/s in a 1:12 twist. Going to a 1:11 twist the bullet is stable at muzzle velocities of 4,000 f/s down to about 1150 f/s, and going to a 1:10 twist make the bullet stable at muzzle velocities of 4000 f/s down to about 500 f/s

Another catch is that Greenhill assumes that the bullet's specific gravity is 10.9 (a lead cored jacketed bullet). For other bullet construction such as a steel core you need to apply a fudge factor by determining the bullets specific gravity. The formula would be:

Twist = [Square Root (10.9 / specific gravity of the new bullet)] * twist derived for a lead core bullet

You can determine the specific gravity of a bullet thusly:

1) Suspend the bullet at its balance point from the pan of a scale.
2) Weight the bullet.
3) Place a container of water under the scale so the bullet hangs fully in the water and weigh the bullet.
4) Subtract the weight obtained in step 3 from the weight obtained in step 2
5) Divide the weight obtained in step 2 by the difference obtained in step 4.

As an interesting exercise you can also determine the spin needed in mediums other than air under "standard conditions" by multiplying the spin for air by the square root of the number obtained by dividing the density of the medium in question by the density of air. As an example water is about 900 times as dense as air: 900 / 1 = 900 and the square root of 900 is 30. Thus you need a twist 30 times as fast to stabilize a bullet in water.

Miscellaneous Questions #2

 

[FLT]

I’ve played with a lot of pistol caliber carbines , both 1 in 10 and 1in16 , with pistol bullets I didn’t see any meaningful deference. I was never satisfied with the accuracy I could achieve with subsonic pistol bullets . To get accuracy that was acceptable to me I had to use better bullets . That usually meant rifle bullets , and they didn’t expand unless you bought something like Lehigh bullets at more than a dollar per bullet. which is ok for hunting but rather expensive to plink with. I’ve recently been experimenting with the new hornady 250 grain subX bullet in a 357 magnum carbine and it’s showing real promise. At about 40 cents a bullet is more affordable and expanded pretty well on the hogs I tried it on.

 

[Baddog 0302]

Seeing 3 types, 1:10, 1:9.85, 1:16.
Glock 9mm g26.
What's the best twist? For 115, 124 or lighter.

It's bullet LENGTH NOT WEIGHT that determines twist, but , unless you want to change barrels ?? and spend $$$$ "run what you brung "

 

[G-rat]

my sbr has a 5.5" Balistic Advantage 1/10 twist barrel. not sure in 5ish inches of barrel it matters tho!

 

[Rebel_Rider1969]

It's bullet LENGTH NOT WEIGHT that determines twist, but , unless you want to change barrels ?? and spend $$$$ "run what you brung "

Building a G26 upper. Was wondering about why there was so many different twist rates on 9mm. Also more reading reveals 1:24 is used. Wow.

 

[DAS HUGH!]

I myself would go for the one offering the max RPMs if you plan to rum hollowpoints. Only because they act like a blender once the claws open up. What the max is for that bullet type in a tiny pistol is tho I dunno.

 

[Rebel_Rider1969]

I myself would go for the one offering the max RPMs if you plan to rum hollowpoints. Only because they act like a blender once the claws open up. What the max is for that bullet type in a tiny pistol is tho I dunno.

Probably use bits or poly propellers.

 

[DAS HUGH!]

Probably use bits or poly propellers.

It's a shame Envirude don't make a ammo ;-)

 

[Rebel_Rider1969]

I went with the 1:16T for my G26 build.

 

[Rebel_Rider1969]

Bought a 1:10 threaded one too.

 

[Jevaughn]

Bought a 1:10 threaded one too.

I'd be really interested in your results with the two different twist rate barrels in the same gun. Keep us posted!

 

[Rebel_Rider1969]

I'd be really interested in your results with the two different twist rate barrels in the same gun. Keep us posted!

The factory barrel is 1:985. So 3 to play with. The two aftermarket ones are rated for lead. 5r.