SG, Twist Rate & Plastic Tipped Bullets

[rcw3]

I need clarification on an issue with plastic tipped bullets and the twist rate of the barrel needed to properly stabilize them.

Is it true that plastic tipped bullets typically require less of a twist rate to have them properly stabilized than if they were not plastic tipped but they had the same overall length?

You have the twist rate calculator here on the Berger site, and for a 22 cal (.224") Hornady 80 Amax (1.160" long) running around 2750 fps (at 59 degrees and at sea level) you need a 1:7" twist barrel to properly stabilize the thing with an SG around 1.5 - here's the calculator on the Berger site http://www.bergerbullets.com/twist-rate-calculator/

If you run it on the Berger calculator with a 1:8" twist barrel you get an SG at around 1.16 which is on the ragged edge of stability.

Then take the JBM ballistics calculator here http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

If you run the numbers for the Hornady 80 gr 22 cal (.224") Amax and plug in the plastic tip at .140" long, with an 8 twist barrel you get an SG of 1.479, which is fine for stability.

So what's right? What's going on here and what do we use to guide us with the new plastic tipped bullets (especially since more bullets are making use of plastic tips)?

Robert

 

[spclark]

Robert (you've been reading that thread over at the NM forum...) I trust this will get settled once Sierra's new bullets get some serious numbers out into our community of folks. Not many I know shoot Noslers, Hornady being the only other brand so far.

In that the JBM calculator includes a function that allows entry of tip-specific data, that'd be my first choice. Plastic is less dense than either copper or lead. To my thinking this means the length added by its use ought not to affect considerations based on length of conventionally constructed bullets, the plastic tips effectively moving the bullet's center of mass rearward with respect to length. Berger does something similar with their column design by careful control of the length of the lead core.

Maybe Berger's calculator is biased towards their (so far) untipped products? That could change; Brian Litz's 'real-world' numbers drive both calculators after all.

 

[rcw3]

spclark

Good Morning & Happy New Year!

Actually what started this whole thing was I have a 1:15" twist 30 cal barrel chambered in a smaller 30 cal cartridge and I wanted to see if the 125 gr. Nosler Ballistic Tip bullets would be good in it. The bullet length is 1.070" and it has a plastic tip that's about .170" long. With the typical calculation using the Berger calculator it's hanging in the "no go" range for stability in a 15" twist barrel (i.e. SG = 1.13), but then I run it through JBM's and it's a definite optimum "go" (SG = 1.551) at the velocity chosen.

So the situation is no one wants to spend the money for a premium barrel, have it all chambered up and then find out the twist rate is wrong for the intended use and potential bullet - - so I guess I am just going through "measure twice, cut once" thing for future edification and would like some authoritative word on the subject.

Be well!

Robert

 

[spclark]

So the barrel's chambered already, huh? What's to risk but the price of a box of bullets? What are your alternate choices if, once you try some of the Noslers, then find they don't quite cut it? I'd probably go with Berger's 115 or 150 FB myself or even the 155VLD unless it's really chilly.

This isn't a target build I assume....

 

[johara1]

Robert, i bit the bullet on 6mm barrels and the Spencer 103 being on the edge of stability in the 8 twist barrels. I went back to my 7.83 barrels and life is good again, they shoot as good in the morning as the afternoon. I tested no less than 9 different 8 twist and all display the same tendencies except for one that was actually a 7.9 twist ……. I'm sure you will be at the same area with the 30……….. jim

 

[rcw3]

spclark

If it is just the one already chambered 15" twist 30 cal barrel I have and that is the last barrel I would ever own - then yes, who cares and what's the difference, I'll just use another bullet.

But what about all the barrels we sell every year to customers who ask things like - "I want that 8" twist 22 cal barrel you have, but I have 2500 80 gr .224" Amax bullets I need to use up and I want to make sure they will work well in the barrel you're selling me" or "I need that 8" twist 6mm barrel right away, but I want make sure the 105 gr 6mm Amax will work well in it". So what do I tell them? Do I say "yeah that will be fine" and then when it's not - "oh well, sorry about that"? That's not good - need to know.

Robert

 

[Phil]

Jim,

How long are the 103 Spencers???

Phil.

 

[Bryan Litz Ballistics]

It's my belief that the formula used on JBM (which accounts for plastic tips) over-predicts stability for plastic tip bullets. In other words, I think that for the case you cited in the OP, stability is probably closer to 1.16 for the 8 twist barrel and 80 grain Amax.

Based on live fire testing, I'm inclined to advise toward faster twist barrels when in doubt, especially for long range applications.

You can have exceptional accuracy with SG's as low as 1.1, but the bullet will be flying with a reduced BC due to the marginal stability. This is why I advise faster twist for long range applications. For short range applications (like 100 yard BR) you can live with low SG and a compromised BC. The advantage of low SG's in the 1.1 range is improved short range precision. By spinning the bullet slower, you're minimizing dispersion effects related to imbalance and mis-alignment. To be clear, it's not a low SG that is the cause of good precision, it's the reduced RPM's resulting in less dispersion.

All of the above doesn't directly address the question of: "Can/should I use or advise an 8 twist for the 80 grain Amax?" My direct answer to that would be: I think the 8 twist will be accurate at least to ~600 yards, but the bullets will be flying with reduced BC and will probably suffer at 1000 due to transonic effects.

Another caveat; if the rifle is planned for use in high altitude only, the 8 twist will produce more stability (due to the less dense air), and it may be good all the way to 1000. It's at sea level that I would worry most.

Hope this helps,
-Bryan

 

[johara1]

Jim,

How long are the 103 Spencers???

Phil.

Phil, they run 1.235…….. jim

 

[Phil]

Thanks Jim.

 

[m500]

Bryan, I have a couple questions about your statement:

All of the above doesn't directly address the question of: "Can/should I use or advise an 8 twist for the 80 grain Amax?" My direct answer to that would be: I think the 8 twist will be accurate at least to ~600 yards, but the bullets will be flying with reduced BC and will probably suffer at 1000 due to transonic effects.

Are you saying this because the sg has not increased to an adequite level to combat the increased stability requirements upon reaching the transonic region?

If you shot a bullet from a barrel at an sg of 1.3 or 1.4 for example, do you have a formula to determine how many yards it would take to reach an sg of 1.5, or more? Obviously this will depend on a number of things including atmospheric conditions and types of rifling groves. Thanks

 

[Bryan Litz Ballistics]

m500,

SG does increase as the bullet flies downrange. The problem is that as the bullet slows to transonic, it needs a static stability (SG) much higher than 1.5 to counteract the strengthening 'dynamic' instability encountered as the bullet slows to transonic.

In other words; at supersonic speeds at the muzzle, having an SG of 1.5 is plenty high enough. But downrange, you need much more than 1.5 for good flight.

Predicting both static and dynamic stability becomes very difficult at transonic speeds. As a result, it's very difficult to know which bullets will 'survive' transonic flight in good form. Things that help are shallow BT angles (7 degrees or less), or no BT at all. In general, short fat bullets are more stable thru transonic, but knowing exactly which bullets will suffer transonic stability effects and how much is beyond the current state of the art in aerodynamic prediction. It has to be determined by live fire.

Take care,
-Bryan

 

[mikecr]

Correct me if I'm wrong, but I believe;
JBM dropped McCoy's math for Miller twist rule a while back. This is a rule of thumb approach(modern Greenhill).
Michael Courtney then modified it to subtract the plastic tip from bullet length input. This, allowing the Miller twist rule to pass more tests with plastic ballistic tipped bullets, which do stabilize where it seems counter intuitive.

I don't understand why the plastic tips don't hurt stability.
Based on reported BC, they don't appear to actually reduce drag, and for a given weight for length, center of gravity should fall even further behind center of pressure(a destabilizing attribute) while including low weight tips.
I could understand it, and expect it if the tips were heavy.
Bryan, can you explain it?

 

[m500]

Bryan, thank you for your reply. I realize sg increases with distance, hence the first question in my earlier post. I agree with your explaination on this. As to my question about the formula, I should have explained it better than I did. I was not questioning the effects during the transonic region, rather the stability before it. This is in regards to the stability calculator on the berger site, which as you know gives a reduced bc based on its sg. What I was looking for was a way to determine at what distance a bullet would reach its full bc value.

 

[r bose]

Mr. Litz, I've always enjoyed reading your explanations of bullet flight, twist rate, and stability data. The science behind it fascinates me. Living in South Louisiana and shooting at or sometimes below sea level, I've always fell on the "safe side" of twist rate. I have a 223, 6.5 twist rifle shooting berger 90's very well but the barrel is at it's end of useful life. I recently purchased a 7 twist to replace my current barrel. I hope I'm not hurting myself by using a 7. I don't plan on shooting past 600 with it.

For long range I have a 6xc shooting 105 hybrids out of a 7.5 TW. Possibly a bit overdone as I've heard 8 TW "will work" but I didn't want to chance it. My point, probably better to give a little more twist than not enough.

 

[Bryan Litz Ballistics]

Mike,

JBM still hosts the McGyro (McCoy) based stability calculator, as well as the Miller formula. The McCoy one is more difficult to use because you need to know more entries, like ogive radius, specific gravity, etc. http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi

The plastic tips have both a stabilizing and de-stabilizing effect and they tend to offset each other somewhat, resulting in little net effect on stability. The stabilizing effect is that the bullet has a higher axial moment of inertia (MOI). In laymans terms, this just means it's mass isn't as spread out along the long axis of the bullet, but more concentrated at the center. This is a property of short fat bullets and is good for stability. The thing about plastic tips that harms stability is that the tipped bullets tend to be longer than non-tipped bullets, and the tip pushed the center of pressure further forward. These offsetting effects tend not to take the stability too far off that predicted by the un-modified Miller formula, which is a ~+/-10% approximation anyway. McGyro typically does better, but there's no way to account for plastic tips in that method.

m500,
I see what you're saying now. That's a good question. From live fire testing, I can tell you that it doesn't seem to matter if BC is measured over 300, 600, or 1000 yards. If SG is 1.3 at the muzzle (for example) then the BC for the entire measured flight seems to be reduced by the same amount. This is likely due to the magnitude of limit cycle yaw (basically a trim angle that a bullet wants to fly with) is affected by SG everywhere. As the bullet slows down and SG increased in supersonic flight, the limit cycle yaw angle is trying to increase. So you have two battling effects as the bullet slows down. Getting SG above 1.5 seems to be the threshold where you don't see much improvement in BC thru supersonic by further increasing spin rate.

If you're interested in a deeper explanation of limit cycle yaw and twist rate effects, check out: "Modern Advancements in Long Range Shooting". The entire first part of that book is dedicated to live fire experiments on twist rate effects.

r bose,
If you go to a 7 twist for your 223, I would recommend trying the 80.5 gr FULLBORE bullet and compare that to the 90's. Let the rifle tell you which it likes. The 90's may be accurate, but with reduced stability the BC may not be any better than the 80.5's which will be fully stable and flying with their 'best' BC. Plus you'll pick up some MV. Just have to throat the chamber so it can work for either bullet considering seating depth.

Take care,
-Bryan

 

[eric32]

What is the effect of a Sg of 3.0+ and beyond?

If i ran a 1 in 8.5twist, 308win with a 175gr to 185gr bullet with a estimated Sg of 3.0

Would it amplify spin drift?

 

[Berger.Fan222]

I need clarification on an issue with plastic tipped bullets and the twist rate of the barrel needed to properly stabilize them.

Is it true that plastic tipped bullets typically require less of a twist rate to have them properly stabilized than if they were not plastic tipped but they had the same overall length?

You have the twist rate calculator here on the Berger site, and for a 22 cal (.224") Hornady 80 Amax (1.160" long) running around 2750 fps (at 59 degrees and at sea level) you need a 1:7" twist barrel to properly stabilize the thing with an SG around 1.5 - here's the calculator on the Berger site http://www.bergerbullets.com/twist-rate-calculator/

If you run it on the Berger calculator with a 1:8" twist barrel you get an SG at around 1.16 which is on the ragged edge of stability.

Then take the JBM ballistics calculator here http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

If you run the numbers for the Hornady 80 gr 22 cal (.224") Amax and plug in the plastic tip at .140" long, with an 8 twist barrel you get an SG of 1.479, which is fine for stability.

So what's right? What's going on here and what do we use to guide us with the new plastic tipped bullets (especially since more bullets are making use of plastic tips)?

Robert

I faced the same question recently when considering whether to try the 55 VMAX in the .222 Rem with a 1 in 14" twist. The JBM calculator accounting for the plastic tip said it would work. The Berger calculator estimated SG at 0.8 which predicts definite tumbling end over end.

Unsure of which to believe, I contacted Hornady, who assured me that the 55 VMAX works fine in a 1 in 14" twist at .222 Rem muzzle velocities. It turns out it works great. I've seen it shoot MA scores at mid-range F-Class on still days.

 

[jim_k]

RCW, I think the answer is to say 1:8 may not work; but BC won't be optimal (or known), and your customer will not know for sure about accuracy or BC, until they do their own testing, which, for your purposes, is too late. Your customer will be either happy or unhappy. The customers who get 1:7 will all be happy, until they light up some 52 gr bullets going/spinning too fast. But those will be highpower shooters, and they can just reduce the velocity to bring the rotation rate back down. They are only shooting 52's at 200 yards, anyway, right? Who cares what the wind drift does. At worst, they would only lose the ability to shoot a bullet under 69 gr., and they all have a truck full of 68-69 gr bullets.

 

[Berger.Fan222]

I'm not sure the "faster twist just in case" approach is always necessary or prudent. I've seen too many cases of bullets exploding en route to the target and too many failed attempts to get lighter bullets to shoot accurately. I don't think equipment with a 40 year life span should be limited by the intended use in the first year or two. In my experience, at some point many owners want to load light bullets for reduced recoil, varmint hunting, or just a change of pace. My advice is to select a twist fast enough for the longest bullet you want to shoot, but no faster.

Many bullet companies (Sierra, Berger, Hornady, etc.) test their bullets in different twist barrels and are most likely the most reliable source of information for the required twist rate under standard (sea level) conditions. If Hornady says the 80 AMAX will shoot well at sea level and provide the proper BC in a given twist, that is good enough for me.

It should also be noted that emerging technologies may soon make competing and conflicting stability formulas a thing of the past. For example, Hornady's 4DOF ballistic calculator provides a SG determination for all the supported bullets with user entered conditions and twist rate. This SG determination uses the same advanced technologies (PRODAS) used by the Department of Defense and that bullet companies pay tens of thousands of dollars for.

At least for Hornady bullets, I expect the new Hornady calculator's SGs to be very accurate. I wonder how its results would compare with the predictions of simpler formulas for the supported bullets under the same conditions?