Can anyone explain why Olympic uses barrels with a twist rate of 1 in 16 when others are using 1 in 10?  Does spinning the bullet slower have any effect on accuracy?   Spikes uses a Green Mountain Chromoly that is 16" with a 1:10 twist.

Even the four Glock 9mm pistols have a 1 in 10 (ok if we round up from 9.84") twist rate.

Help me out here....why so slow?  What does it affect?  Good or Bad?

Thanks,
HardRider

I can ring an 8" plate pretty much 100%
at 100 yards with my 16" Oly, standing, offhand,
so the twist rate must be OK. It is for me,
anyway.

John

The faster twist (1:10) barrel will have more drag and friction, which can slow the bullet down.
Ideally, you want to use the slowest twist rate possible that will stabilize the 9mm bullet.
A pistol length barrel needs to be able to stabilize the bullet within a shorter length than the 16" barrel of the carbine, so the faster twist is needed.

If you wanted to SBR your carbine, then maybe the 1:10 barrel would be better.

Also, the heavier and slower bullet will need more stabilization than the lighter, higher velocity bullet.
So a +P 9mm should love the longer 16" (1:16) and will give the round more velocity than a pistol could.

In contrast, a 45acp carbine that is SBR'd might need a 1:10 twist for better accuracy.

X

Here is a twist rate calculator.

LINK HERE.

The 147gr bullets I have on hand are 0.662 inches in length.  The diameter is .355.  The last time I chrono'd these bullets, they went 1050 FPS.  (A bit Hot, I know, but that's what my Glock likes, and they're still sub-sonic for suppressor use. )

When I plug this into the equation, then the calculator spits out 16.9"

I don't know if this is scientific enough or not, but it makes me wonder why we put the 1:10 twist rate in the rest of the 9mm barrels.  I don't know the answer for that......

I think the 1:10 rate is over-stabilizing the pistol calibers a little, which would cause a bit of spin drift. But since it's not really a sniper weapon it probably does not matter. I think the main point is that you loose the ability to gain some additional velocity out of the longer 16" barrel., assume the rounds are on the hotter side.

This was interesting:

http://en.wikipedia.org/wiki/Heckler_&_Koch_MP5

H&K switched from 1:10 in th 9mm to 1:15 in the 40/10mm round for the MP5. Looking at the calculator, it appears that the bullet length vs diameter proportions do come into play.

X

Ok, that makes sense.  I'm wondering how the Montana Gold 124 grain will work.  I'll take some measurements tonight and run them through the twist calculator (thanks for the link) and post the results.

Thanks for all the help.  Much appreciated.

HardRider

Not to hijack the thread, but can you help me understand that calculator?  For s&g's, I put in a typical 9mm pistol projectile but gave it a muzzle velocity of 3000fps, and the calculator gave me a twist output number of 40.  Would that indicate a 1 in 40 twist rate?

I've got the same problem. The Montana Gold 124 grain bullet (tip to bottom) is .5940 and the diameter is .355.  I'm using a conservative velocity of 1090 fps and a S&G of 10.9 (normal for copper jacketed lead) and it tells me it needs a twist of 24 with an error displayed of "low muzzle velocity"!!!

Confused....

HardRider  

The Greenhill formula was developed in 1879 and is more a "rule of thumb", rather than a "be all, end all" calculation.  There are certainly some barrel and gun makers that believe in super slow twist rates for 9mm barrels as calculated by the Greenhill fromula.  I have seen mentions of 9mm bullseye guns with slower barrel twist rates of 1:16, 1:24, and even 1:32.

But, Glock and HK use a 1:10 twist rate for their 9mm barrels.  I would have to imagine that a lot of development and testing was done before they both settled on that twist rate...

I don't know if 1:10 is needed, but the faster twist rate is supposedly to stabilize the 147 - 158gr bullets at subsonic velocities.

I have noticed that some manufacturers are using 1:10" for 9mm lately, and I can't understand why they would do that. 1:16" and 1:18" are standard twist rates in 9mm, .38 Special, .357 Magnum, .38 S&W, etcetera and they stabilize 200 grain bullets down to 650 fps or less. In fact, my NEF .357 single shot with a 1:18" twist stabilizes 280 grain roundnose cast bullets at subsonic velocities. And my .35 Whelen , which was designed for bullets to at least 300 grains, is 1:14". So I can't see any possible reason for a 1:10" 9mm. The 9mm load that wouldn't stabilize in a 1:18" barrel would neither fit in the magazine nor cycle the action.

I seriously think somebody fell asleep at the switch and ran a shit-ton of 9mm blanks with the wrong rifling twist, and they are being foisted off on a public that accepts everything and questions nothing.

Man, Colt done fucked up for the last 30 years with their 9mm carbines...

If only their guy didn't fall asleep at the switch... you think they would have pulled his dead body off that switch by now...




If only someone could trade their Oly barrel for my unreasonable POS barrel, anyone? Please.








ETA: not everyone uses 1/10 or 1/16... I once had a M1S barrel in 1/15.
16' C.A.R. 9mm Chrome Moly Lightweight Barrel, 1-15' twist, 1/2 x 36 threaded muzzle

Yeah, well, Colt uses 1:7" in all their 5.56 barrels, too. And the Python always had a .355" groove, 1:14" twist barrel. Works well for slow, 148 grain soft lead wadcutters, but probably not the best for a max velocity .357" diameter 125 grain JHP.

So what were your results like with the 1:15 barrel?

Thanks

Since when are 1:16 and 1:18 "standard" twist rates for 9mm?  If you believe it at face value, the greenhill formula would seem to indicate that an ultra slow 1:24 to 1:32 - giver or take - would be the "standard" twist rate for 9mm (assuming 115 gr as a standard bullet at around 1200 fps).

It would be an interesting test if someone were to source 4 barrels in 1:10, 1:14, 1:24 and 1:32 twist and compare them at 25, 50 and 100 yds to see which groups better with various ammo.  Anyone have a few grand sitting around burning a hole in their pocket?

!/10" barrels go all the way back to the Sten gun, if not further.  Back then it was probably a tooling issue.   Certainly, the 9mm does not need that tight of a twist for accuracy.  

At some point it may have been decided that a fast twist rate barrel helped hollow point bullets to expand more rapidly.

For long range accuracy, a 1/16" twist would be much better, but how many people shoot 9mm at long range?

Probably not a lot of  people.  However I have plenty of room on our new place so I intend to shoot mine out to at least 100 M if not a little bit further depending on how many new houses go up around us.