Need consistency with recommended twist rates

[rcw3]

Berger 6mm 105 gr. Hybrid target bullet:

Recommended twist by Berger on box and published literature 1:8" (I thought this data came from Litz).

Recommended twist in Berger's Twist Rate Calculator in many normal and reasonable shooting situations 1:7" (I thought the calculator also came from Litz)

Recommended twist in Litz publication Ballistic Performance of Rifle Bullets 1:7.5"

Customers who are accuracy hounds (and we tend to border on being a little OCD to begin with) are constantly wondering and fretting if the 1:8" twist barrels they purchased are going to shoot well.

Where do we go with this and why can there not be some consistency?

Is a 1:8" twist adequate or not?

 

[Bryan Litz Ballistics]

Robert,

The inconsistency that you're pointing out is the result of there being a lot of gray area when it comes to stability.

Stability depends a great deal on your environment. A shooter at 5000 feet above sea level (asl) can stabilize the 105 Hybrid with a 1:9" twist, but a shooter at sea level needs a 1:8", and maybe a 1:7.5" in the winter.

Velocity affects stability also.

Then there's the question; how stable does a bullet need to be? The 1:8" at sea level will stabilize the bullet and you can shoot great groups but the BC might be a little depressed because the stability factor (SG) is under 1.5. To get all the BC out of the bullet you might need the faster twist.

So one approach would be to recommend the twist rate that will fully stabilize and get all the BC out of every bullet in any condition. But such a conservative approach would leave many shooters thinking they can't shoot a bullet when it might actually work just fine for them. In the case of the 105 Hybrid, the conservative recommendation would be 1:7.5" but many shooters have won many matches with this bullet from 1:8" barrels which may or may not have been giving the bullet 100% stability in their environment.

Funny that I saw your question today because this was one of the things we (Berger) were going to discuss in a call today. What should our recommended twist rates be based on? Should we show the number to the 0.1", 0.5" or whole numbers? What environment should we assume? How should we word the twist recommendation so that it's; accurate, and not misleading?

The approach I like is to bracket it. For example:
Minimum twist required for stability: 1:8"
Recommended twist for optimal performance is 1:7.5" or faster

In this case, the 'minimum twist' would be based on achieving an SG of 1.2 at sea level, and the 'optimal performance' would make an SG of 1.5 which is where you fully get all the BC. This scenario still doesn't account for the effects of altitude.

The truth is, when selecting bullets or barrels, you need more information than what can be conveyed in a single number.

The recommended twist number is a bare bones indicator of twist requirement.

The Berger twist rate calculator will give you the greater detail you need to really make well informed decisions.
http://www.bergerbullets.com/twist-rate-calculator/

Take care,
-Bryan

 

[rcw3]

Hi Bryan

My personal impression is that the twist rate calculator has not exactly been helpful to us since it tends to over estimate the twist rate needed for stability with the 105 Hybrids (i.e. it suggests a twist rate tighter than is reasonably needed and I am getting readings suggesting the need for around a 1:7" twist rate in situations that are not extreme or worst case situations). Our customers sometimes go on there and punch up numbers to check their prospective or actual barrel purchases and come back to us sometimes a little huffy and/or testy thinking we're doing them a dis-service because we did not recommend and/or do not sell 1:7" twist 6mm barrels.

Robert

 

[Bryan Litz Ballistics]

Hi Bryan

My personal impression is that the twist rate calculator has not exactly been helpful to us since it tends to over estimate the twist rate needed for stability with the 105 Hybrids (i.e. it suggests a twist rate tighter than is reasonably needed and I am getting readings suggesting the need for around a 1:7" twist rate in situations that are not extreme or worst case situations). Our customers sometimes go on there and punch up numbers to check their prospective or actual barrel purchases and come back to us sometimes a little huffy and/or testy thinking we're doing them a dis-service because we did not recommend and/or do not sell 1:7" twist 6mm barrels.

Robert

Robert,

You say the information isn't helpful but it's specifically answering the questions you had in your initial post. It may be inconvenient that people are asking for different twist barrels, but considering what we've learned about stability, they're not wrong.

It's very clear in the live fire test data that BC improves with increasing twist rate until SG reaches 1.5. This reality is modeled in the twist calculator to allow shooters to see what twist is required to maximize BC.

Slower twists may compromise BC a little or a lot without harming precision (groups) at all so it goes un-noticed by many shooters. Noticed or not, the effects of depressed performance are real.

The twist rate calculator isn't always 100% accurate, but it is representative. I certainly wouldn't change my barrel that's shooting good because of it, but I might rebarrel to a faster twist next time. For what it's worth, my .243 barrels are 1:8"s for shooting the 105's. I shoot them pretty fast and in summer conditions; if the BC's are depressed it's not enough to worry about in my application (1000 yard prone).

In summary, the twist calculator gives you good information to make decisions and understand the 'gray area' of stability a little better. It's not perfectly accurate but it is pretty good. Heck the atmoshperics change from morning to afternoon each day in some places by an amount that's equivalent to 1/2" of twist anyway so it's not wise to specify twist to the third decimal to 'optimize' it.

Hope this helps,
-Bryan

 

[rcw3]

Bryan

I'm not "busting" on you but there are also practical considerations, and 7" twist 6mm barrels are a bit of a "sore spot" with me. A number of years ago when the 115 DTAC's came on the scene, an 8" twist was deemed to be hanging "on the edge" twist wise for the 115's. At that time I obtained two 7" twist 6mm barrels (one cut rifled and one button rifled, both premium barrels from top makers) and two of my friends also obtained 7" twist 6mm premium barrels. All of us had pressure problems with all the 7" twist barrels, even with the 105-107 gr class of bullets (which we resorted to after we had pressure issues with the 115's). My one friend became so frustrated with his that it he took it off his rifle with a pipe wrench and gave it to me (pipe wrench marks and all) for additional testing with the two I already had that were problematic. The issue was/is that when working up loads, or in matches where strings were fired without a break, even with moderate loads (i.e. like a rapid fire or even a slow prone match) pressures would climb and spike quickly, unpredictably and uncontrollably and accuracy would also suffer. I have never had those issues with a 7.5" or 8" twist 6mm barrels, and I know of no one who has. So when a 7" twist 6mm barrel comes up as a recommended barrel twist, and customers start pressing because the twist rate calculator says a 7" twist is recommended, yet I won't recommend it, that's where "the rubber meets the road" and sales can be affected. So that's a part of where I am coming from.

Robert

 

[Joe Salt]

Robert my buddy just got a 6MM 1-8" twist for his new light gun and that was the recommendation from a lot of the 1000 yard shooters. We broke it in friday, got rained out saturday, but sunday he shot 8.5" ten shot group with a 96 score. He was using the 105 VLD's so you may want to try some of them. I've never tried reloading for a Dasher, but a little playing around I think I got it. I know guys that have good luck with the Hybrids but not consistently! Its like the 30 Cal. I stick with the 1-10'' twist It will shoot most any bullet.

Bryan you coming to the World Open? Haven't seen you since 2000 Open!

Joe Salt

 

[rcw3]

Joe Salt

Not going to disagree with you at all that the Berger 105 VLD's (and the Berger 108's) are definitely "steady Eddie" consistent performers that run real well from 8" twist barrels.

Robert

 

[Bryan Litz Ballistics]

Guys,

I checked the bullet length that's stored in the stability calculator; some of those numbers are old.

It has a length of 1.290" for the 105 Hybrid. We've been measuring recent lots to be more like 1.256" long.

Length is an important parameter for stability. Changing this input to the more accurate number of 1.256" shows that a 1:8" twist will make a stability factor of 1.48 in standard atmospheric conditions, which means that in most 'summer match' shooting conditions and locations, the SG will be comfortably above 1.5 most of the time. Winter conditions would put the SG further below 1.5 and cause more severe stability and BC depression..

I'm always harping on users of software to check and double check inputs, and I made the mistake myself

I guess the lesson is to consider the default values for bullet length to be approximate when using the stability calculator, best thing is to measure lengths of your actual bullets to get more accurate numbers. We'll (Berger) look at updating the numbers that are stored as well.

Regarding your bad experiences with 1:7" and 1:7.5" twist, I wouldn't blame it on the twist rate. There's no reason why an inch faster twist would cause the problems you were seeing. What could cause those problems is copper build up which would be more of an issue with the long bearing surface bullets you would be shooting out of faster twist barrels. Some barrels build copper more or less than others for reasons I don't understand but I don't see how twist rate could make a barrel foul and build pressure worse. Poor finish maybe, but you could have that in any twist rate.

I've got fast twist (1:6") barrels we use in the lab for testing long monolithic bullets in 6mm and 6.5mm. These barrels are not noticeably less accurate than the 1:8" or 1:10"s when using quality bullets. Granted we don't typically shoot long strings with them in match conditions but if inherent accuracy is the question, there's not a problem. Fouling is another issue.

In general, high performance bullets for long range shooting are growing in length because longer heavier bullets have higher BC's. To get the most out of these long bullets, twist rates have to evolve and get faster as well. We've also learned that BC can be compromised if you don't get a stability factor of at least 1.5, so that's another factor driving twist rates to the faster side. We can't get stuck in the mentality of 'whats always worked', because things are changing. Palma shooting is another example that's seen it's bullets evolve, and with that evolution the twist rates have moved from 1:13" to 1:12" to get max performance out of the longer higher BC bullets.

There are countless other examples; Nosler Long Range Accubonds, Barnes LRX BT's, all these are longer bullets which won't fully stabilize in conventional twists anymore. Rifle makers need to recognize these changes and adapt or they'll be depressing the performance of rifles with twist rates that are too slow to stabilize the highest performance bullets.

In the case of the 105 Hybrid though, we agree the 1:8" is sufficient in most summer match conditions.

-Bryan

 

[Joe Salt]

Bryan you are right about Copper build-Up! That's why before I fire a shot threw a new barrel, I JB the crap out of the throat to smooth out the machine marks. And keep doing it though the break in. Oh and for those against it I also use Flitz and Kroil. And my barrels do not copper up. Robert I think a lot of people get the Idea that they have to use a certain bullet because everyone else is having good luck with it, but you have to find what works in your rifle. I know guys using the Hunting VLD's that are having great luck, I tried a box but no cigar! Its the same with powder, try something new if its not working. But please ONE CHANGE at a time so you know what done what.

Joe Salt

 

[rcw3]

Joe & Bryan

IMO - The pressure issue is a twist related issue - - I did way way too much testing with the three 7" twist 6mm barrels I have (tons of time and $ wasted figuring that one out). I would NEVER buy another 7" twist 6mm and would NEVER recommend one to a customer or anyone else. 7" twist is the "point of no return" with a 6mm barrel IMO (o.k. for a 22 cal barrel, and o.k for a 300 Blackout or a subsonic 30 cal., but not for a high velocity 6mm shooting bigger conventional type pills). With a gain twist barrel I am sure it would be o.k, but not with a 7" twist right where the bullets hit the lands. Simply stated, at some point a steeper ramp becomes more like a wall and with a high velocity 6mm, a 7" twist barrel is right there shooting the big conventional projectiles.

Joe - - How many bench guys are shooting 7" twist 6mm's? Probably ZERO, and if they are shooting 7" twist barrels, how many are winning with them? I bet the number is about ZERO.

Robert

 

[Joe Salt]

Robert I think you are right don't know of anyone using a 7" twist they are mostly 8''. Have you tried any 5c barrels I think you can get away with more pressure using them from what I hear, And I have an old Obermeyer that is a 5R that my buddy who cambered it told me he didn't have a pilot small enough so he had to buy another to fit, and asked me to let him know how the pressure was. So I took the fattest bullets I have and ran them throw the barrel, to my surprise I could even go higher with the load. Maybe because of the round groves.

Joe Salt

 

[rcw3]

Joe

What blows me away is the rest of the shooting community seems somewhat oblivious (more like in denial) of the fact that a twist rate can become too fast for an application such that it causes disproportionate excess pressures (i.e. with the 7" twist barrels I hit a point where pressures suddenly rose disproportionately). Originally I was not accepting of this, yet the situation repeatedly presented dramatically in the three 7" twist barrels I had and still have. BTW - - the other friend who had the fourth 7" twist barrel had the same issues. He used his for high-power rifle competition and essentially dealt with it by shooting only very mild loads until he ultimately shot out his barrel. For rapid fire sequences with the 7" twist barrels, I would typically start out a rapid fire string just fine, but toward the back end of say 10 shots, and even with a moderate load, by the end of the string I could barely manipulate the bolt because of the excessive pressure, the primers would be dead flat and I would have to wrestle with the rifle to get finished the 10 shots - - and because of the excess pressures at the back end the accuracy went from great to horrible - - useless - totally useless barrels.

FWIW - I also have a Broughton 7.3" twist 5c barrel and that barrel is not as bad as the others but it definitely exhibits shades of the same situation, but to a much lesser degree than the 7" twist barrels. I also never had these issues with any of the numerous 7.5" and 8" twist 6mm barrels I have shot and competed with over the years (conservatively 30+ 6mm barrels).

So the short of it is - no more 7" twist barrels for me, whether they're 5c (or 5r) or not.

Robert

 

[Joe Salt]

That's what I would recommend 8" twist. Never heared for Bryan whether he could make it to the World Open. Probably busy Making Bullets!

Joe Salt

 

[dcali]

What blows me away is the rest of the shooting community seems somewhat oblivious (more like in denial) of the fact that a twist rate can become too fast for an application such that it causes disproportionate excess pressures (i.e. with the 7" twist barrels I hit a point where pressures suddenly rose disproportionately).

How did you measure the pressure? I'm a little skeptical that a 7.5" barrel will be fine but a 7" won't.

But I can imagine a situation with very aggressive twists where the bullet jacket starts to yield axially (pushing the bullet straight down the bore rather than spinning), as opposed than inline with the twist (what we want), which I suppose could cause pressure jumps. But I have no idea when (or if) that would happen.

I saw someone at the range blowing up 90's (.224) with a 6.5" twist (might have been 6"). So it's not totally crazy to assume that we might be approaching some sort of limit with these really short twists. But given how well my 7.5" 6mm shoots, I'm a little doubtful that the limit is at 7", at least not universally.

 

[mikecr]

Well nothing is free. Else barrels would only come in 6tw & ~20".

I avoid excess twist because of torque and pressure and jacket blowups.
I agree that it's better to have high Sg rather than low, but only to a point before the balance swings opposite.

 

[rcw3]

How did you measure the pressure? I'm a little skeptical that a 7.5" barrel will be fine but a 7" won't.

But I can imagine a situation with very aggressive twists where the bullet jacket starts to yield axially (pushing the bullet straight down the bore rather than spinning), as opposed than inline with the twist (what we want), which I suppose could cause pressure jumps. But I have no idea when (or if) that would happen.

I saw someone at the range blowing up 90's (.224) with a 6.5" twist (might have been 6"). So it's not totally crazy to assume that we might be approaching some sort of limit with these really short twists. But given how well my 7.5" 6mm shoots, I'm a little doubtful that the limit is at 7", at least not universally.

As to the pressure question - - Pressure signs, like flat primers, primer pockets being expanded out in one firing so the brass becomes useless, sticky bolt lift - - you don't need a pressure gauge to know what you're seeing, it's obvious.

No disrespect, but I have been down this road many times before and it always winds up the same - - a plethora of posts from people speculating why my findings cannot be true (usually always posts from people who never actually tried a 7" twist 6mm with a high velocity cartridge pushing the big conventional 6mm projectiles). Your post is respectful and inquiring and I thank you for that, but many prior posts on this issue (not in this thread - Bryan and Joe Salt have been first rate here) have not been and some have all but accused me of fabricating this for some improper motive. The truth of the matter is that in my business I also sell a lot of 6mm barrels (Bartlein, Krieger, Brux, Hart, and Pac Nor) and it doesn't matter to me what twist people want, we can stock and can sell them anything they want, but I am not going to stock and sell them barrels when I know darn well they're going to be potentially problematic - - that's just bad business and not right for the customer either.

My best advice if you still doubt is buy a couple 7" twist 6mm's and chamber them up in something like a BRX, a Dasher, a 6XC, a 6mm Creedmoor, a 243 Win, a 6mm Super LR, etc. and try them yourself. I can say from personal experience it's a real bummer to go through all that (and spend all that money) only to realize you have a bum barrel because of its twist rate.

Robert

 

[Joe Salt]

Robert do you have a Web page for your business? And thanks for what you said. I went through all that stuff when I had my buddy purchase his barrel,asked a lot of the top shooters and they all said 1-8" for the 6mm. I'm a 30 cal. guy and know that a 1-10" is the way to go. And I'll set right alongside anyone with one of my rifles and show them how they shoot.

Joe Salt

 

[dcali]

Unless I'm hearing something I know can't be the case, I try to keep an open mind. So I'm not doubting your observations, just trying to think out loud about what the explanations could be.

If you imagine a rifle with a 1/4" twist, I'd bet dollars to donuts that the bullet would not spin very well if at all. I'd guess pressure would be high, and accuracy low. This is total speculation, but at some point the rifling is too steep to do it's job, so I think it's reasonable to assume there is a limit to how fast we can spin a bullet. I would imagine that the exact limit would depend on the yield strength and thickness of the jacket material among other things. Again, just speculation. *If* that is true, then you *might* see a marked increase in pressure after the jacket hits it's limit.

(Enough weasel words for ya? )

Now, take my 6XC with a 7 1/2" twist. It shoots fine. Well, even. So somewhere between 7.5" and 0", there is a point where things fall apart. It sounds like you're saying that it might be at 7". I have nothing to refute that, as I've never shot a 7" twist 6mm. If others have shot them and not seen that effect, then that would suggest that you're seeing something else, or that the cutoff point is not so precise (perhaps it depends on the bullets).

In any case, I had always assumed that the limit was too low (fast twist) to be of practical concern, but maybe not. It's only recently that people have really been pushing twist rates (at least in my lifetime - I'm sure this stuff is cyclical to some degree), and so maybe we're just now getting enough interest in super-fast twists to start to see these effects in detail.

 

[Joe Salt]

damoncali I don't know what distance you are shooting, but thing really come to light at 1000 yards. But I think this has been said before a bullet has to spin at a certain rate, the perfect spiral so to speak. So are all barrels made to the advertised twist, I doubt it. So that may be part of the trouble. Do some people get hummer barrels that no matter what they put through it shoots. YES! I've seen guys with all kinds of problems getting their rifles to shoot. When all they had to do was change powder, bullets, or primers. Or a matter of seating depth. So all I'm saying is ask some of the top shooters in your discipline. With your 7.5 twist you may be at the magic number for what you shoot.

Joe Salt

 

[mikecr]

For a while I shot a T2K in 6XC & 7.5tw, like most with this cartridge. If I went to 7tw(like some)I would expect that I'd have to abandon my stiff 7.5tw load as pressure would stop me from getting back to it anyway.

Why 7.5tw to 7tw as a problem, like a step change?
I think the combination of bullets and powders that take us to this edge to begin.
If you review the efforts of folks with 22-243s and 90VLDs(orig), you'll see they put themselves into a narrow window between sufficiently stable and failure.