14" twist for 30-06 rig

tetan

tetan

Regular
Rating - 100%
31   0   0
Some of the most popular twists for .308 Palma shooting are 1:12" and 1:13". Wouldn't it be logical that for a same application one has to use at least 1:14" for .30-06 (155s bullets)? What would be an optimal twist for .30-06 30" barrel if one wants to shoot bullets not heavier than 175gr?
 
Last edited:
1-14" twists work well for .308 Palma rifles (and they will stabilize a Sierra 168 and Sierra 175 too). So a 1-14" would definitely work for a .30-06 shooting 155s-175s.
 
When light bullets are loaded there is little velocity difference between the .308 and the .30/06. What works in the .308 for 155s will also work in the '06. I would be hesitant to go with a twist slower than 1:12. From an accuracy point of view, the quality of the barrel means much more than the twist, provided the twist is fast enough to stabilize your bullet of choice.
 
Boomer's right. Tuning twist (using as slow as is possible) is a very, very subtle thing; you'll only see the effect, if you even see it at all, after you've done *everything* else to the nth degree. Only once you're running absolutely top notch gear and ammo, is it even worth worrying about, and probably not even then. I would certainly second his recommendation to get the very best 1-12" (or 1-11", or 1-10") twist barrel you can possibly afford.

FWIW, one of the top US Palma shooters (John Whidden) used a 1-10" twist, shooting 155s, and did extremely well with it (i.e. win matches, set national records, etc)
 
While I have some 14 twist 308's, my choice if buying a new barrel is always a 12. A 30/06 with a 26 inch barrel will push 155's to 3100+fps quite easily and 3200 is not out of reach. If velocity won matches it would be a winner for sure. Regards, Bill.
 
Thank you ALL kindly for your responses and advices!

rnbra-shooter said:
...Tuning twist (using as slow as is possible) is a very, very subtle thing; you'll only see the effect, if you even see it at all, after you've done *everything* else to the nth degree. Only once you're running absolutely top notch gear and ammo, is it even worth worrying about, and probably not even then...
Click to expand...

Are you sure that overstabilizing bullet and therefore bringing up all unwanted oscillation harmonics (none of the bullets are perfectly balanced) is going to have such neglectable small affect on accuracy?

I was also trying to find some study that would theoretically or experimentally show a time of flight vs RPM of bullet. Something tells me that RPM is going to be lost not linearly with loss of speed but rather somewhat faster (interaction of bullets surface with environment). Have you seen such work/study?
 
Time of flight has nothing to do with the RPM of the bullet.
1 in 10 is the standard for .30 cal bullets. A real Palma rifle is a heavy barreled, single shot, weighing 12 to 14 pounds.
 
tetan said:
Are you sure that overstabilizing bullet and therefore bringing up all unwanted oscillation harmonics (none of the bullets are perfectly balanced) is going to have such neglectable small affect on accuracy?
Click to expand...

Real world experience (i.e. people getting very good accuracy with "faster-than-necessary" twists) shows that very good accuracy can be gotten. Remember that I didn't say that it didn't matter, I said that in comparison to the "big things", it didn't matter. So, be sure to have a top-quality barrel, properly installed on a good action, properly installed (bedded, usually) in the stock, firing good bullets, from well made ammo. If you're not doing all of these things "right", and you are having accuracy problems, it's almost certainly the fault of one of the things you're missing, not an overly-quick barrel twist.

I was also trying to find some study that would theoretically or experimentally show a time of flight vs RPM of bullet. Something tells me that RPM is going to be lost not linearly with loss of speed but rather somewhat faster (interaction of bullets surface with environment). Have you seen such work/study?
Click to expand...

I haven't seen bullet RPM decay rates well studied and quantified. Many people are surprised to learn though, that bullet RPM decays quite a bit more slowly than forward flight speed. This means that the bullet actually becomes *MORE* stable as it flies down range.
 
rnbra-shooter said:
I haven't seen bullet RPM decay rates well studied and quantified. Many people are surprised to learn though, that bullet RPM decays quite a bit more slowly than forward flight speed. This means that the bullet actually becomes *MORE* stable as it flies down range.
Click to expand...

It is very interesting observation! So what you are saying is that bullet actually get more gyroscopically stable as it fly down the range (lets not to worry about a point when bullet goes subsonic and begins to tumble)???
Do you happen to remember a source where I can read about this phenomena a bit more?
 
tetan said:
(lets not to worry about a point when bullet goes subsonic and begins to tumble)
Click to expand...

As a bullet goes transonic it may go through a zone of instability. Or it may not.
I've seen the 500 grain bullet that leaves the muzzle of a .45-70 at 1200fps still shooting sub-MOA at 300m. I'm willing to bet that it's gone subsonic well before the target. At no distance that we shot targets was there any evidence of keyholing.
 
Sorry tetan, I don't know where to send you for a reference and for more reading on this. Are you looking for a second source for this (wouldn't blame you!), or an understanding of why it is that this happens?

Yes, the bullet gets gyroscopically more stable as it slows down.

(caveat - as it slows to transonic speeds (M=1.2-1.4) and slower, for many bullet designs the overall stability might start to decrease because the overturning pitching moment increases / the centre of pressure moves aft).

You might hear that bullets destabilize and tumble when they go subsonic. Or that they lose accuracy when they go through transonic and become subsonic. Those things can happen to some bullets, but they certainly don't happen to all bullets. Black powder shooters regularly show good results with highly subsonic bullets. And although highpower shooters generally try to avoid subsonic loads like the plague, sometime loads end up being subsonic, and sometimes very good scores end up getting shot with long pointy bottail bullets arriving subsonicly.
 
Dan is bang on. Bullet stability is relative to RPM. The "sound Barrier" is not the ballistic obstacle that some perceive it to be, and most bullets remain stable throughout their flight. Subsonic bullets still function perfectly... look at the majority of pistol ammo. No 230 grain 45 ACP bullet ever spent a millisecond in supersonic flight.
 
If you do a search on benchrestcentral.com, some rocket scientists (they said they really were) did a bunch of dopler radar testing and covered a variety of these queries.

What I gathered was that spin stability increased as a bullet slowed down. That can be bad sometimes because it can keep the bullet tip up as it falls - more drag.

Also, bullets that tumble when they go subsonic, are affected by their original design as it relates to aerodynamic balance. you can set a bullet design to work great when supersonic but not so great when it goes subsonic. There is no solution for these bullets to make them fly well when they go below the speed of sound.

The aerodynamic centers of balance go out of whack and the bullet tumbles.

They covered a variety of things, some of which flew over my head like a 747 but it is there for the reading. you can also contact Eric Strecker of Berger Bullets. They just hired a ballistics engineer to test all their bullets to get better BC numbers. There is a wealth of info available for the asking.

Ultimately, I leave it to holes in paper and smaller groups are better.

Jerry
 
Also consider high velocity (barely supersonic) or hypervelocity (fairly supersonic) .22 LR ammo, fired at 100 yards. It starts out supersonic, but it is well subsonic at 100 yards. It'll likely shoot like crap at that distance (at least it always has every time that I've tried!), but the bullet holes will be round.

W.r.t. rocket scientists, don't pay them too much heed, they're a dime a dozen really.... ;-)

If you read "Hatcher's Notebook" (highly recommended!), he did some interesting experiments firing a .30-06 machine gun (1-10" twist) vertically into the air.

When he used 150 grain flat base ammo, the bullets were so overstabilized that they returned to earth still pointing nose-up.

When they fired 173 grain FMJBT ammo, the bullets were stable but not nearly so overstabilized. At the peak of their trajectory (~10,000') some would tumble and some would not. So the bullets returned in two batches - first, the boattail-down ones, followed by the tumbling ones. (the still-stable boattail-down ones had less drag than the tumbling ones, so they got back sooner).

Berger recently hired as their chief ballistician Brian Litz, who is a pretty sharp cookie (and rocket scientist to boot). He also shoots a Palma rifle quite well.
 
You must log in or register to reply here.
Back
Top Bottom