What is the right twist rate for a .308 and why?

[Terryw123]

So I am in the process of building my first precision rifle and I am trying to just put faith in the company that is building the rifle for me. I am trying to wrap my head around barrel length and twist rate. I have a Remington 700 .308 and it will be sitting in a McMillan A4. Originally I was looking to make a M40A3ish clone but, the company I am working with is recommending a 21" 1:10 twist. I trust the company and I really didn't want to bother them with 50,000 questions. I was looking at different builders and trying to figure out why the 1:10 twist rate vs 1:12 or 1:11.25 what helps decide the twist rate of a barrel? Is there any relationship to length and twist rate? Any info put into basic concepts would be greatly appreciated. Most of the articles I was reading were a bit over my head. Thanks!!!!


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[mdmp5]

1 in 10 gives you latitude for a wider range of bullets on the heavy side. 1 in 12 was pretty standard for a long time, then guys started getting interested in bullets heavier than 175 gr in a 308. Also, a 1 in 10 may work better for shooting subsonics, which usually run on the heavier side for better performance. You can get away with shooting 200 gr subs in a 1 in 10, but very iffy in a 1 in 12.

 

[Terryw123]

What is the right twist rate for a .308 and why?

So if I am just shooting factory match loads and nothing suppressed is the 1:10 the right twist rate? Right now I have about 200 168 hornady TAP. I'm by no means stuck on that but since I don't reload yet I want something that will fit my needs.


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[LocoGringo]

It depends on the size of the bullet you plan on shooting. If you plan on shooting the heavier 190+ grain .30 caliber bullets, you should probably use the 1:10 twist. A 1:11.25 twist rate is just about ideal for 168-175 grain bullets. I'm not really sure what the 1:12 is "ideal" for except maybe the 150 grain and below. However, if you're stuck with a 1:10 twist rate, it won't hurt the medium weight (168,175) bullets. That's what little I know. I'm sure someone with more experience will come along and explain things better.

Oh, a suppressor has no relationship with twist rate except to say that you MUST stabilize the bullets sufficiently before they exit the muzzle to make sure you don't get a baffle strike. Now, subsonic vs. supersonic is relevant.

 

[diderr]

1:10.....

 

[Terryw123]

Thanks diderr


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[johngfoster]

MontanaMarine has a thread here about shooting 208gr A-MAX bullets out of his 1:12 308. They seem to work fine for him. I use a 1:11 twist on mine and that works great for 168 and 175gr bullets. The tighter twist will certainly stabilize the lighter bullets, but you may end up with higher pressures trying to drive the bullets through more rotation before exiting the barrel. This may be insignificant, or it may be the difference between blowing up your rifle if you are at the edge of the envelope with your load.

 

[redneckbmxer24]

1:12 will stabilize anything up to and including the 208 Amax, it has been proven over and over. While spinning a bullet faster doesn't hurt it also doesn't really do much, a stable bullet is a stable bullet. What you do get with a slower twist is more velocity.

 

[alman1531]

If your choice is 1-12 and 1-10, your deciding factor should be are you shooting subsonic loads. If yes than deffinatly go with 1-10, if not than flip a coin and either will work fine.

 

[Barikade]

It depends on what bullet you want to shoot and how long it is. Longer (Palma) barrel run slow twist because of their additional velo (its RPM's not twist rate in itself that stabilizes, so if you shoot a bullet faster through a slower twist you ARE increasing its stability factor) As barrels get shorter, twist rates tend to increase to maintain a nice buffer or safe zone. Palma shooters also run on the ragged edge of stability to maintain a really low Angle of attack delta when that .308 round is falling out of the sky at 1k. Think of a football QB with "great touch" what they mean by that is he changes how hard he grips the laces depending on the distance he is throwing to, so the ball indexes "point" down and stays rotating about its flight axis and doesn't belly first "sail" down. As far as I can tell applying this to long range shooting is mostly a voodoo hypothesis at this point but a lot of champions swear by it. In my .308 with 20 inch barrel if I could have my ideal twist it would probably be 1:11.5 or thereabouts. Plenty of safety for my 178 Amaxs and abit more speed and less erosion than my current 1:10.

 

[JB02]

I think that a 1/10 is best suited if you want to drive heavies >200gr. 1/11.25 is the perfect twist for ~ 175gr. If you plan on shooting mainly 155s, go with a 1/12.

If your rifle builder only has 1/10 twist barrels in stock, there is nothing wrong using one.

 

[Graham]

The tighter twist will certainly stabilize the lighter bullets, but you may end up with higher pressures trying to drive the bullets through more rotation before exiting the barrel. This may be insignificant, or it may be the difference between blowing up your rifle if you are at the edge of the envelope with your load.

I defy anyone to be able to measure the pressure difference between a 12-twist and a 10-twist.

No way are you going to blow up a rifle due to pressure issues caused by a faster twist. That's a myth.

 

[Rob01]

As stated if the smith only has 10s then get it and go. I have used a few 10 twists and they shoot 168 and 178s just fine. I used to have one companies 12 twist which wouldn't stabilize 178s unless they were pushed above 2675fps. It was a button rifled barrel and wasn't a true 12 twist. Also the reason i use Bartlein cut rifled barrels now.


10s or 11s are what I would use unless you plan on shooting 155s all the time then a slower will be fine.

 

[wilwith1l]

Bryan Litz had an interesting comment on .308 twist rate 3 days ago:

Faster than necessary twist rates have many effects, but most are subtle.

Something like a 1:8" .308 Win shooting 175's can produce very slightly higher BC's thru supersonic (on the order of 1%), and noticeably higher BC's thru transonic (I've measured greater than 5% difference going from 1:12" to 1:8"). The effect isn't so dramatic in ballistic performance, as in you'll see any less wind drift, but where it can matter is with drop predictions being off. But before you consider this a major issue, ask yourself how much shooting you do thru transonic.

Another effect is on precision (groups), based on how well balanced the bullets are. You won't see groups affected by twist so much for well balanced bullets, but for poorly balanced bullets you can see noticable growth in groups with faster twist. Over a range of 1:8" to 1:12", I've measured group averages (again, 308's with 175's) that ran from like .9" to .6" for poorly balanced bullets, but for well balanced bullets the average only ran like .6" to .5" (something like that, I'd have to check my notes to be sure on the exact numbers).

Someone brought up a good point about the effect of run-out in fast twist. That makes sense also as misalignment would cause more dispersion in faster twists. I'll have to run a test on that as well.

Some people worry about losing muzzle velocity with faster twist. Testing shows this just isn't the case, at least not at a level that's significant. I think it was like 1 or 2 fps per inch of twist.

-Bryan

 

[sirhrmechanic]

In am with Graham on this... the pressure is never the issue. It's a confluence of bullet weight, velocity and twist that matters.

The Sako TRG that I shoot as a duty rifle is a 1:11" and it's the most accurate rifle I've ever used. 1:10 and 1:12 are also popular with .308's and as long as they are matched with a good bullet weight and a good load. In which case, either will be fine and either are as good or better than the best rifles out there. Thus I would argue that there is no 'perfect' twist rate for a .308. Merely a 'perfect' bullet and velocity combination for the twist rate on your rifle, be it 1:10 - 1:12... or something beyond either end of that spectrum.

I can say that my duty TRG utterly loves 168's. It will also shoot lighter and heavier bullets very well. But if I want to wring the best repeatability from it, the 168 is it's favorite. BH Match hollowpoints, to be specific. It does not like AMAX tips as well, even when they are BH Match. There is a definite difference. But that's MY rifle. Yours may be different. Even if it's a TRG.

Once you reach a certain point, it comes down to individual load development and the characteristics (or foibles) of your own gun.

Cheers,

Sirhr

 

[Graham]

What is the right twist rate for a .308 and why?

Yup: I shoot 180 long boat tails in a 12-twist, no issues, even in winter. So, it's not just about barrel twist.

 

[Barikade]

People keep talking about weight here, my 155 Scenar is only .020 shorter than my 178 Amaxes and its significantly longer than any 168 HPBT I'm aware of. I have not measured any >170 grain VLD's as they may exceed my Scenars average of 1.273 The more aggressive the BT the more twist you would need as the bullet is getting longer without the same proportional increase in weight.

 

[Graham]

What is the right twist rate for a .308 and why?

The more aggressive the BT the more twist you would need as the bullet is getting longer without the same proportional increase in weight.

A boat tail does not add to the bearing surface.

 

[42N8 1]

I have a 1:11 but I only shot 175s

 

[mdmp5]

Here is Berger's twist rate stability calculator. I am sure there are many exceptions to the rule, but the most important thing this calculator teaches is the effect that twist, bullet weight, bullet length, caliber, and velocity all have on the stability factor. Used for nothing more than an educational tool or a confirmation. I wouldn't outright be dissuaded from shooting a bullet by the calculated stability factor alone, but it would lend reassurance or caution if I were shooting suppressed.

http://www.bergerbullets.com/litz/TwistRuleAlt.php

 

[shoot4fun]

I've shot mostly 175 SMK in 308 pushed by 44.7 Varget for the last few years. My barrels have been 10, 11.25 and 12 twist and I have seen only negligible differences in velocity, long range stability and accuracy.

 

[Barikade]

A boat tail does not add to the bearing surface.

Is bearing surface length the only consideration for stability? Not asking a rhetorical question here, it has been my understanding that the total length of the projectile was what determined it's required stability factor.

 

[alman1531]

The total length does have a big impact on the stability factor. Generally longer bullets does mean more bearing surface, but not always (ogive and boat tail length). You need the weight and length to determine how much spin you need to properly stabalise the bullet. Just like a top that is very tall will need to spin a lot faster to stay up than one that is short due to the center of gravity being higher. This is not a perfect analogy but it does help explain what is going on.

 

[Greg Langelius *]

A 1:10" twist is basically a good generic twist for .308.

What I would do is to tell my builder what I wanted the rifle to do, and let them select an affordable barrel to do the task. They can then work with their suppliers to get the best specs, price, and delivery time. When the rifle ships, the builder can then provide you with the better recommendation on bullets and load ranges.

In the end, the part we depend on most from our builders is their good judgment. I know for a fact that my judgment cannot be as good as an experienced builder's, their experience tops mine by several times over.

Greg

 

[Terryw123]

Thanks everyone. I feel a lot better about the process and I have a little better understanding on what I am purchasing. I have a long road ahead of reading and learning.


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[Barikade]

The total length does have a big impact on the stability factor. Generally longer bullets does mean more bearing surface, but not always (ogive and boat tail length). You need the weight and length to determine how much spin you need to properly stabalise the bullet. Just like a top that is very tall will need to spin a lot faster to stay up than one that is short due to the center of gravity being higher. This is not a perfect analogy but it does help explain what is going on.

Yes that was my understanding as well, I'm not quite sure why we started talking about bearing surface though. The reason I brought up the weight thing, was that people can get caught up talking about weights (generally a heavier bullet is longer) they forget that lighter bullets (like a scenar or VLD) with their radical BT's and multi radius secant Ogives, can be considerably "longer" than they are heavy, thus requiring a faster twist than what is assumed based solely on the average length for a given weighted bullet with a more traditional geometry.

 

[Graham]

What is the right twist rate for a .308 and why?

Hmmm.... There is no such thing as something 'longer than it is heavy', like something 'faster than it is large': Makes no sense.

VLDs don't need a faster twist because they are pointy. How long can you get a bullet without increasing its bearing surface?

Longer bearing surface in the same caliber means a heavier bullet. That's how weight goes up when caliber stays the same.

Assuming that the caliber stays the same, you can't have increased weight without more bearing surface: A long boat tail might add five grains in .30 cal without increasing the bearing surface, but then you won't need to change the twist rate in order to stabilize it.

 

[wilwith1l]

Assuming that the caliber stays the same, you can't have increased weight without more bearing surface: A long boat tail might add five grains in .30 cal without increasing the bearing surface, but then you won't need to change the twist rate in order to stabilize it.

I'm confused. This seems contradictory. You say "you can't", but then you provide an <S>anecdote</S> hypothetical in which "you can".

Edit: I don't know how to strike through.

 

[The Gump]

Needed twist rate is directly related to the total length of the bullet, not the length of the bearing surface.

 

[Graham]

What is the right twist rate for a .308 and why?

Point is: Increasing the weight without increasing the bearing surface won't get you very far.

What's the necessary twist rate for a PIAT round??

 

[Barikade]

Yes, adding more bearing surface will increase the weight more than an equal amount of additional length on an ogive or BT, that's fairly intuitive. What I am saying however, is that you can increase the non-bearing surface length of a bullet to a point where a faster twist would be optimal, staying within the exact same overall weight category. a 168 SMK, depending on it's metplat, will average about 1.205, a 168 Barnes triple shock will measure 1.315. I specifically chose the Barnes example because it's solid copper, which again will require a faster twist to optimize than what would be assumed based solely on it's weight. While both of these will easily stabilize in probably every single common .30 cal twist rate, my point was that looking just at weight or "bearing surface"would be technically incorrect. Another example is current issue 5.56mm M856 tracer ammo. That round weighs in at 63.5 grains and requires a 1:7 twist to properly stabilize, because it's so long A typical length .224 bullet @ 63.5 grains would stabilize just fine out of a 1:11. I myself, have shot M856 out of a 1:9 Bushy and had keyhole strikes at 25 yards. I know these are extreme examples.

 

[Graham]

What is the right twist rate for a .308 and why?

Yes, adding more bearing surface will increase the weight more than an equal amount of additional length on an ogive or BT, that's fairly intuitive. What I am saying however, is that you can increase the non-bearing surface length of a bullet to a point where a faster twist would be optimal, staying within the exact same overall weight category. a 168 SMK, depending on it's metplat, will average about 1.205, a 168 Barnes triple shock will measure 1.315. I specifically chose the Barnes example because it's solid copper, which again will require a faster twist to optimize than what would be assumed based solely on it's weight. While both of these will easily stabilize in probably every single common .30 cal twist rate, my point was that looking just at weight would be technically incorrect.

Copper is less dense and lighter than lead. So wouldn't the Barnes have a larger bearing surface at the same weight?

 

[Barikade]

In this instance yes, however not necessarily. For instance a 155 Scenar is longer than a 175 Smk and has a shorter bearing surface, same with the new 2156 155 MK palma match bullets. Technically, they would be optimally stabilized at a faster twist than the old 175MK, it is heavier, shorter, and has a longer bearing surface.

 

[Graham]

Lighter bullets of the same construction in the same caliber have less bearing surface and use a slower twist is the rule. Your example follows that rule.

 

[Barikade]

The 155 Scenar, and 2156 MK are lighter, longer overall, and with a shorter bearing surface require a faster twist to optimally stabilize. I am saying you can require a faster twist with a shorter bearing surface with some of the very long and lighter bullets.

 

[Pusher591]

I'm not an expert, I don't reload (yet), and I'm really a nobody but I use to shoot a 17.5" shorty 308. It was a Brux 1-11.25 barrel. I shot mainly Hornady 178 bthp reloads over 44 grains of Varget and routinely saw 2550-2580 velocity speeds. I shot this rifle at 730 yards on so many occasions I lost count. It always held MOA and mostly less then MOA. I even made 3 hits at 1050 with it on 22"x22" steel. As confirmed they were not keyhole hits either.

Mainly shot at around 400ft above sea level in temperatures from 40-110 degrees.

I don't know if that helps but that's been my experiences. I did opt for a 1-10 twist on my new .308 build because I'm very intrigued by shooting a 308 with 185 and 190 projectiles if I can get them to 2650 out of a 25" barrel. That should make for a great back up gun in case my 6.5 ever goes down.

 

[Graham]

With less than 20" of barrel in a .308 I would go with a 1 in 10 twist.

 

[Terryw123]

Thanks punisher. I think it's gonna workout for my needs. I was a little worried about length but shoot if your lobbing rounds to a 1000 out of a 17.5 I shouldn't have an issue hitting 900 with a 21"


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[Pusher591]

Thanks punisher. I think it's gonna workout for my needs. I was a little worried about length but shoot if your lobbing rounds to a 1000 out of a 17.5 I shouldn't have an issue hitting 900 with a 21"


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I will say this though, those 3 rds that hit was out of a 11 shots, I should have been more clear. If you plan to compete then velocity is your friend, especially with a 308 but if your leisurely shooting for fun you won't have any trouble at 1000 so long as you can read wind fairly well.

We've extended our range now and have 860 yards to play with and we shoot 18" 223's at that distance pretty often, as well as 308's and we don't have any trouble hitting 9" wide steel.

 

[Terryw123]

Yeah I think at my local range range the furthest targets are at the 860 range. As long as my rifle can cover that I will be happy. All my issues come from my lack of experience with precision shooting. I was an 0321 with 1st Recon Bn and we had 2 snipers in my team. So for me the logical place to start was a M40A3. I'm not married to the specs but, I didn't understand why the M40A3 is a 25" barrel with a 1:12 twist. However the company I am using to build my rifle was recommending a 21" 1:10 twist. I figured was good for the Corps was good enough for me to get started. What I have been told is that I'm not really gaining that much more velocity with the extra 4 inches. If I'm looking for a rifle that's gonna be a 1000+ yard gun consider a different cartridge. So I can kinda buy this theory by just looking a ballistics chart of a .308. It almost becomes indirect fire at those ranges. As for the twist rate, it seems as the 1:10 is a fairly standard twist rate for a .308 these days. It also allows for heavier bullets to be fired. So that is my basic understanding to this point. Please everyone correct me if I am wrong. Thanks for the input.


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[Terryw123]

What is the right twist rate for a .308 and why?

Pusher how long is your barrel on this rig?

http://www.snipershide.com/shooting...les/104946-why-stock-you-have-your-rifle.html


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[The Gump]

Yeah I think at my local range range the furthest targets are at the 860 range. As long as my rifle can cover that I will be happy. All my issues come from my lack of experience with precision shooting. I was an 0321 with 1st Recon Bn and we had 2 snipers in my team. So for me the logical place to start was a M40A3. I'm not married to the specs but, I didn't understand why the M40A3 is a 25" barrel with a 1:12 twist. However the company I am using to build my rifle was recommending a 21" 1:10 twist. I figured was good for the Corps was good enough for me to get started. What I have been told is that I'm not really gaining that much more velocity with the extra 4 inches. If I'm looking for a rifle that's gonna be a 1000+ yard gun consider a different cartridge. So I can kinda buy this theory by just looking a ballistics chart of a .308. It almost becomes indirect fire at those ranges. As for the twist rate, it seems as the 1:10 is a fairly standard twist rate for a .308 these days. It also allows for heavier bullets to be fired. So that is my basic understanding to this point. Please everyone correct me if I am wrong. Thanks for the input.


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You're correct. I have a thing for the M40-A3 but the barrel profile is too heavy and too long in my opinion. 12" Twist should work but 10" is more ideal for heavy stuff in short barrels.

Just because a gun is speced and used by this group or that doesn't make it the best. Just sayin...

 

[Terryw123]

Thanks 816 I totally get that just because Marine Corps snipers use it doesn't make it the best. For me it's all about a starting place and getting educated to make a sound purchase on limited funds. I feel as though the more I post my questions the more you guys educate me since I didn't score a quota for sniper school. I just don't remember that rifle being that heavy or awkward but, let's face it I'm not as good as I once was!


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[The Gump]

Thanks 816 I totally get that just because Marine Corps snipers use it doesn't make it the best. For me it's all about a starting place and getting educated to make a sound purchase on limited funds. I feel as though the more I post my questions the more you guys educate me since I didn't score a quota for sniper school. I just don't remember that rifle being that heavy or awkward but, let's face it I'm not as good as I once was!


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Understood. I look at it from the standpoint of "I may shoot this thing off hand." It's kind of a heavy gun for that IMO. I'm no body builder though.

 

[Greg Langelius *]

If this topic has reached the point of including additional chamberings, then I'm game.

I no longer own any .308's. I replaced them with .260 Rem's, and have recently added several .280 Rem's.

IMHO, these two (.308 and .30-'06) .30 caliber chamber capacities are better served with smaller bore diameters; 6.5mm for the '08 and 7mm for the '06. They can achieve the same BC with lighter weight bullets, and better BC's with similar weights. The net benefit is longer and/or flatter trajectories.

The Savage Predator Hunter Max ! weighs 8 1/2lb with a 24" barrel, but no optics, and is capable of 1Km with the right loads. It's a lot more wieldy in Offhand, too.

Greg

 

[Pusher591]

Pusher how long is your barrel on this rig?

http://www.snipershide.com/shooting...les/104946-why-stock-you-have-your-rifle.html


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That was the 17.5" Surgeon was referring to. The barrel was cut/crowned at 17.5" but I added the SF brake. That particular brake over overlaps the barrel mostly, but still adds about 1.4" to OAL.

 

[Terryw123]

Greg I really haven't gotten into different calibers. Again mostly an education thing. Are those rounds easily available factory loads? I won't be reloading for a couple years but I save all my brass for the day it comes.

It's a nice rifle pusher thanks for the info.



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[Greg Langelius *]

First, I would hesitate to call any chambering easily available these days.

Then, I would suggest that both .260 and .280 are less available than, say, 7-08 or .270. The .260 and .280 are definitely logical candidates for hand reloading, while 7-08 and .270 may be better served by factory ammunition. Our advertiser and site supporter SW Ammo is heavily into producing excellent .260 ammunition.

Hornady .280 Rem Superformance 139gr SST loads are outstanding, and several brands of .260 ammunition are available.

I've been handloading the .260 for over a dozen years, and can confidently report that I have not had any difficulties using 120-142gr bullets and H-4350 that might be attributed specifically to the .260. It is a chambering that can very effectively use additional barrel lengths longer than 24".

I have not been able to develop and handload the 280 as extensively, but again, no special issues. Accuracy is actually pretty easy to find with 120, 140, and 150gr Ballistic Tips propelled by H4350 and H-4831. This is another chambering which benefits from longer barrels.

Both are easily capable of 1000yd with confidence, I have shot the 260 very successfully at 1Km, and would venture that the .280 is still going strong passing that distance.

Greg

 

[Terryw123]

Nice Greg how do they compare to a .300 win mag? I was considering that because I have a buddy that is reloading it.


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[Greg Langelius *]

If you are looking to match the trajectory of the 300WM, the .260 works. If you're looking to equal the terminal energy of the 300WM, some .30-'06 loads can just barely match it, and the .280 Rem delivers the energy inherent in the '06 case capacity more efficiently than the '06 does.

Greg