Too much twist for bullet weight ?


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dirtykid
March 22, 2013, 05:44 PM
Picking out a new barrel for REM-700 30-06,
common choices are 1:10 or 1:9
I would like to be able to launch 200gr + projectiles at times, but for the most
part will be loading up 150-165gr pills by hand,
Is there such a thing as "overtwisting" ?

That is, the 1:9 twist rate actually de-stabilizing a lighter weight round ?
Could it be compensated for by lower velocity charge ? :confused:

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jstein650
March 22, 2013, 06:17 PM
A 210gr Berger VLD (pretty long bullet) would need ~9.7" at 2500fps. Certainly a 1:9 would stabilize pretty much any .308 bullet. I'm no expert, but I don't really think overstabilization is much of an issue per se. They have noted the tendency at really long range for the bullet to want to stay parallel with the bore axis, even though it should be nosing along the curved trajectory on the way back down after midrange. With very light varmint bullets, like a TNT, blow up could be an issue, and I'm pretty sure the faster the twist, the higher the pressure needed to overcome it. But, I'm sure I'll be corrected if I'm off, but in that range of bullets for the '06, I wouldn't worry about 1:9 being too fast.

dirtykid
March 22, 2013, 06:29 PM
When you say "blow up" a very light pill,
You mean it could shed its jacket on the way out ?

dirtykid
March 22, 2013, 06:31 PM
So my thinking of slowing it down to reduce the twist transferred to the bullet is just the exact backwards of what it should be, I would want to shove it thru the barel faster to minimize the amount of time it is contact with the rifling ?

jstein650
March 22, 2013, 06:37 PM
Yeah, the centrifugal force (say a 125gr thin JHP) is significant when pushed to 3400fps or so. They can just turn themselves into a grey 'streak'! :D Those TNT's have a max velocity indicated on the box, IIRC it may be 3400fps. At any rate, this is a rather extreme example. Not gonna happen with heavier bullets, or any medium game bullet.

jstein650
March 22, 2013, 06:39 PM
No, going slower would be the answer. Barrel contact time is not the problem, it's speed, and centrifugal force on that screamin' little bullet.
(I got this reply out just after your reply #4)

Ridgerunner665
March 22, 2013, 07:59 PM
1 in 10" has been working all this time...even with 220 grain bullets.

No more twist is needed...

dirtykid
March 22, 2013, 11:10 PM
No more twist is needed...

What if I wanted to shoot this rifle 800-1200 yards ?
Would the increased tiwst-rate come into play then ?

wgaynor
March 22, 2013, 11:15 PM
Anytime you increase the distance you will magnify the smallest effect upon a bullet.

kyhunter
March 22, 2013, 11:24 PM
I have shot 150, 165, 165, 168, 180, 190, 208, and 220 grains out of my factory 1 in 10, before settling on a weight. The only thing 1 in 9 MIGHT help for would be 240 or 250 grain pills. Which wouldnt be ideal for long range shots anyway. YMMV

dirtykid
March 22, 2013, 11:30 PM
Thanks Kin-tucky !

There sure are some helpful folks on here !! :)

CSC_Saint
March 23, 2013, 12:00 AM
According to Todd Hodnett in the Future of Twist Rates in 2013's issue of Sniper, twist rates were determined by dividing 150 by length of the bullet in calibers. Later on 180 was substituted for 150 for velocities higher than 1,800 fps. This guy has found through testing that faster twist rates help not just stabilize heavier rounds better, but also help give slightly higher muzzle velocities and thus less bullet drop farther out. All this without signs of overpressure. Worth a read as it kinda shows how the gun industry is kinda short-changing us a little. Now for the average shooter this isn't really going to be much help if your only shootin a couple hundred yards/meters. But if your pushing 1,000 plus, then might be worth your time if your into heavier bullets with higher BC.

MrCleanOK
March 23, 2013, 12:04 AM
Just as an academic exercise, (muzzle velocity)x720/(inches per twist) = RPM.

So that pill from post #2 would need to be spinning at least 185,500 RPM to be stabilized.

Jim Watson
March 23, 2013, 01:17 PM
CSC_Saint refers to the Greenhill formula which was empirically determined for rather blunt projectiles in the light artillery class in the 19th century. It will get you in the ballpark but I would not make a purchase decision with it. There are more sophisticated methods.

The Miller stability formula is used by the Army, Berger Bullets, and the JBM ballistic website.

Berger thinks an 11 twist is adequate for their 215 gr .308 VLD, and a 10 twist for their 230 gr Hybrid.

Their Miller rule calculator says the 10 twist would do for the 240 gr SMK, too.

So I think a standard 10 twist would be fine, but if you want a large safety margin, the 9 will not likely hurt performance of the 150s. I wouldn't expect it to do well with a maximum loaded 125, though.

I can tell you for sure that a 6.5 twist .223 meant for 90 grain bullets will blow up a 75 gr A-max in midair if loaded to maximum. Very accurate with a milder load, though.

MachIVshooter
March 23, 2013, 01:25 PM
1:9 in a .30-06 should be fine for pretty much any bullet & load. I wouldn't opt for it if I planned to shoot a lot of 110 or 125 gr pills, but it doesn't sound like you do.

The only time I've ever seen bullets come apart were some 45 gr. Hornet bullets a buddy of mine loaded in his .22-250. They just don't have the integrity to spin that fast, and not one of them made it to the target.

jstein650
March 23, 2013, 01:28 PM
Just a P.S: The idea of what those fast twist barrels do just blows my mind. We're talking zero to something like 320,000 rpm in maybe a millisecond?!

Art Eatman
March 23, 2013, 01:33 PM
I know that 1:10 can be sub-MOA with 110-grain in the '06. Around one MOA with 80-grain pistol bullets at roughly 4,000 ft/sec.

I don't see where 1:9 would be any problem with 110-grain, and I know 1:10 is fine for sub-MOA with 180s, so the 1:9 would possibly be a tad better, I dunno.

MachIVshooter
March 23, 2013, 01:45 PM
Just a P.S: The idea of what those fast twist barrels do just blows my mind. We're talking zero to something like 320,000 rpm in maybe a millisecond?!

.17 Rem 1:10 twist, 4,350 FPS MV, 24" barrel.

Velocity is reached in 460 microseconds. It's not linear, of course, but there's really no way to calculate the acceleration curve.

RPM: 313,200

Rate of acceleration (averaged): 9,380,000 feet per second, per second.

However, the majority of the velocity is attained in the first few inches, so the initial rate of acceleration is much, much higher.

jstein650
March 23, 2013, 01:54 PM
I was kind of using a linear approach to 3000fps for simplicity, but you're right, most of that speed is reached much quicker!

stubbicatt
March 23, 2013, 02:37 PM
Just a P.S: The idea of what those fast twist barrels do just blows my mind. We're talking zero to something like 320,000 rpm in maybe a millisecond?!
Yep. Here lately I've been watching slo mo videos on youtube of rifles and they all rock to one side under recoil. I suggest it is the twist imparted to the rifle by the mass of the bullet being "spun up" while traversing the bore that does this. Heck even rifles fired on bipods lift one leg while under the forces of recoil.

Equal and opposite reaction and all that... :)

dirtykid
March 23, 2013, 03:51 PM
Well the lightest pill I have planned for this -06 is 165 ,but from what I have researched,
a 180-210 will be good out to 700-800 yards,which is the longest shooting range I have access to around,without driving a couple hours
The twist rate is same price,so I think I will go with 1:9 ,,in the 24.5" barrel, it only
makes a difference of a 1/4 of a turn overall, and Im hoping it will give those pills on the
heavy end, a little extra stability

Thanks for everybodies thoughts!

Vern Humphrey
March 23, 2013, 05:55 PM
For lightly constructed bullets with thin jackets fired at high speed, the bullet can disintegrate when it leaves the muzzle. This mostly happens with rifles in the .220 Swift category. It will not happen with a .30-06. Nor will accuracy suffer.

Art Eatman
March 23, 2013, 10:47 PM
dirtykid, FWIW, I got one MOA at my 500 yard range with Sierra 180-grain SPBT Game Kings. I wasn't really trying for "absolute best" with the last ten rounds I had, that day. 0.8 MOA with careful shooting with the 165 HPBT Game Kings.

Match Kings might well do better.

MachIVshooter
March 23, 2013, 11:20 PM
Here lately I've been watching slo mo videos on youtube of rifles and they all rock to one side under recoil. I suggest it is the twist imparted to the rifle by the mass of the bullet being "spun up" while traversing the bore that does this.

Nope. I did a write up on this awhile back, using a .45 ACP (4-1/2 times the bullet weight of a varmint rifle) and a 1911 (1/4-1/3 the weight of a typical rifle). I don't have the sheet in front of me, and it took some arithmetic, but trust me-the torque on the firearm caused by the bullet's spin is absolutely miniscule.

The way a gun recoils is a mechanical function of the shooter's skeleton. Right handed shooters, the muzzle will twist up and left. Southpaws, it twists up and right. The vector is the same, whether the gun itself has right hand or left hand rifling.

Check out some of your firearms; if you have a decent sized collection, chances are there's a gun or two with lefty rifling. Go shoot it, and you'll find it recoils the same way as one with a RH twist.

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