308 win twist rate ?'s

[ADIDAS69]

What are the negatives of too fast a twist rate for 308 Win. I'm replacing my stainless 24" AR10 barrel and have seen that a couple barrel makers offer a 1:9 twist rate.

[Scorch]

Most factory rifles come with a 1:10" or 1:11" twist. That way you can shoot just about anything you want, up to 220 gr. Now, that said, I don't know anyone who shoots 220 gr bullets out of a 308, so I see no value in having that available. But what you gain from a fast twist is highly stabilized bullets. The USMC snipers' M40A4 uses a 1:11..25" twist optimizing the 172 gr Sierra Match King bullets they use, and the Marine Corps snipers know a thing or two about rifle accuracy Biggest thing is buy a good barrel, don't get the cheapest one you can buy.

[ADIDAS69]

Solid copy re not going cheap. I'll be using 190 grn hpbt, all the literature i can find suggests that the primary negative for high twist rate is with lighter projectiles spinning too fast. To a much lesser extent apparently poor quality projectiles can be destroyed by the cetrafugal forces involved.

In terms of quality if you have any suggestions I'd be happy to hear them. I am looking for 24" 416R QPQ aka salt bath finish.

[Scorch]

I always use either Lilja or Bartlein for bolt rifles, but started using X-Caliber barrels for ARs about a year ago. Nitriting does not seem to make a big difference as far as I can tell, mostly a selling point. Yes, the barrel lasts a little longer, but sometimes for the same cost you can buy another barrel.

[ADIDAS69]

Metallurgically my understanding is that the salt bath is a compromise between chrome lining and nothing. Nitride retains more accuracy but isn't as durable as chrome. Manufacturers probably love it for ease of process compared to chrome. X-Caliber offers a "cryo" treatment. Have you tried it and notice a difference in performance?

[dakota.potts]

In regards to nitride vs. chrome, it's also important that a nitride bath treatment changes the makeup of the metal itself down to a certain point (like case hardening) where as chrome lining is a plating process. The chrome plating can chip and once it starts to chip, the process gets worse. I have not experienced this but I have friends who have had it happen to them.

[Scorch]

Well, reducing accuracy loss due to erosion in a 308 is a non-issue. 308 barrels last 10,000 rounds or more with little accuracy loss. I see little accuracy loss over thousands of rounds in my customers' rifles. But if you want nitriting, go for it. Nitriting makes the surface of the steel very hard, which can reduce friction-induced wear. But if you ever need any barrel work done, nothing can touch the steel once it's nitrited, you throw it away and start over. Also, I have seen barrels warped by nitriting, supposedly since it is done at close to 700 degrees.

Chrome lining is a losing proposition as far as I am concerned. It is mostly for erosion control, and unless you are really cranking rounds downrange or shooting tracers, it can be a negative since it can reduce accuracy.

[Unclenick]

In the late 1990's, Precision Shooting published some tests done by Kevin Thomas when he still worked at Sierra. He incorporated some cryo-treated barrels into their routine bullet testing in machine rest guns. He found .308 stainless barrels lasted about 3500 rounds without cryo treating and lasted about 4000 rounds with it. The process increases wear resistance in stainless tools about 20% and that's more or less in line with what Thomas found. The effect on chrome-moly steel is greater, but Thomas didn't test cryo-treated chrome-moly; just untreated chrome-moly, and those lasted closer to 3000 rounds, IIRC.

Keep in mind that this is a tight accuracy standard. They could shoot about 1/4 moa with their special held-back lot of 168 grain SMK's and watch fliers begin to occur (the first sign of a shot out bore) very precisely. Shooters often can't tell the first few from wind or their own errors. Instead, the "fliers" might open groups to a little over 1 moa and stay that good for a long while.

I have heard Marine Scout-snipers report much longer life on their M24 5R barrels. However, as this NRA article points out, they are much more interested in having a platform that keeps its zero through thick and thin than in having a benchrest knot-driller. They can be over 1 moa and still meet specs.

So this is a case of accuracy being subject to expectations. This Shooting Sports USA article gives the .308 a bit more life (4500 rounds) than Thomas did, but I suspect that is partly a matter of shooters not being able to tell as precisely when best accuracy is really gone.

G. David Tubb suggests he can double the life of his barrels with his throat refinishing system. His accuracy standards are going to be more like those published by SSUSA. I don't know how that would play with a nitrided barrel. I don't expect it would do much with chrome lining.

The other thing cryo-treating is supposed to do is provide some stress-relief to barrels that weren't stress-relived originally. I've read that it does provide a bit, but its small relative to what you'd get from proper oven stress-relief. If you want a barrel that doesn't walk as it heats up, I would be looking at making sure the blank was properly stress-relieved before the barrel was contoured. This is most important for button rifled and hammer forged barrels, where process residual stresses are high.

Regarding barrel twist, this is a bullet length issue. Look at the bullet maker's recommended twist for the bullets you are using and stick pretty close to that. Extra spin causes two things. One you already mentioned, which is bullet disintegration. This usually begins with core stripping, which is when a bullet is spun up so quickly that the lead core starts to slip inside the jacket while it's still in the barrel. Even if such a bullet doesn't disintegrate, the uneven mass distribution that results will open groups up badly.

The other is the effect of bullet runout in the finished cartridges becomes exaggerated. If the bullet tilts even slightly in the bore, since modern pointed shapes have their center of mass (CM) ahead of the center of the bearing surface, tilt takes the CM off the bore axis. This results in drift off the ideal trajectory line. Where r is the length of the offset of the CM, the rate of drift is 2 pi times the spin in revolutions per second times the value of r. So if you have a CM a quarter of a thousandth off the bore line and the bullet is going 2500 fps, you have:

drift rate = 2 × pi × 0.00025 in × 2500 ft/s × 12 in/ft / 9 in

(Note that I am using 12 in/ft instead of the familiar 720 formula because I want revolutions per second, not per minute.)

That works out to 5.24 inches per second. So if the bullet flies 100 yards in 0.12 seconds, it will drift off the point of aim by

5.24 in/s × 0.12 s = 0.625 inches

So, if you randomize the bullet offset orientation in the chamber, then you get a group that is 1.25 inches wider than it need be. If you had 12" rifling, the addition would come down to 0.94".

So, pick your bullet and get the rifling to suit and not extra spin if you can avoid it. Some military barrels, like the 7" twist in an M16/M4 barrel is for stabilizing tracer bullets, which are long for their weight. But their accuracy requirements aren't stellar.

[ADIDAS69]

Unclenick, solid response I appreciate your having actually read all previous comments. That's unfortunately rare.

I'll go ahead and pay for the cryo cause I can. Are the barrel manufacturers you like re proper stress relief? Nosler's published tests are in 26" 1:10 barrels but they don't specify if that's best or just what they chose.

I own an M24 and they are magnificent barrel wise. I had to send it back to Remington twice for trigger replacement cause apparently sh** triggers are the order of the day over there. I finally replaced it with a Huber Concepts trigger. I'm at about 4000 rounds and it holds 1/2 MOA at 400 meters with me behind it. It does better strapped to a lead sled.

The next super special add on I was looking at is carbon fiber. Any thoughts re pro con?

[Unclenick]

I know Douglas and Lilja properly stress-relieve their button rifled barrels. 1100°F in a vacuum oven, IIRC. Any of the vendors who have match accuracy reputations to uphold will have done it if their process makes in necessary. Barrels that are cut rifled undersized and then hand lapped don't need it. Boots Obermeyer invented 5R rifling and did the military barrels on contract, so if you want 5R you could do a lot worse than to buy from him. Krieger makes 5R for most .30 cal.

The other factor in throat wear is the ammo you use, of course, but if it were to turn out the sloped sides of the lands and the lower bullet compression force the 5R configuration actually keeps barrels going longer, that would be a piece of information worth sharing. If you have a borescope, you can often put a cell phone camera up to it and get a decent image anymore. It would be interesting to know how your M24 throat looks now. If you get another 5R barrel and have a borescope, you could track its aging.

I put Timney triggers in my Remingtons. It just seemed best to get out of Remington triggers altogether by using a proven product.

I've not got any carbon fiber barrels yet. I saw several at the NRA Annual Meeting show this year, though. The idea is fine. Ultra-light barrels have traditionally been pretty whippy. The rigidity of cylinders is exponential to the 4th power with radius if the hollow center remains the same size, so you lose a lot of rigidity quickly by removing material. The idea with the carbon is to have a thicker cylinder without adding weight, and the carbon fiber is stronger than steel on a pound-for-pound bases, so you can theoretically get ahead on weight with it. Just don't expect it to be equal on a volume-for-volume basis. You won't get much out of a thin OD carbon fiber barrel. But that's not what I am seeing offered, so the makers must be well aware of all that.

Bottom line, if you have need of a lighter gun, like a scout rifle or a hike-out rifle of some kind, I would go for it and see how you like it. What I don't know is how it compares to building an aluminum barrel tube that lets you tension the barrel with a nut. That, I know, can improve accuracy by limiting bending quite a bit. But it's extra complexity and may not get you down quite as far in weight. The carbon fiber seems more user friendly in that regard. Obeys the KISS principle better.