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Downside To SG = 1.5 - Exploding Bullets?

ThunderDownUnder

I shoot F-Class Open downunder and have always been one to accept and use new technology if it could lead to increased accuracy and score.

I have very much appreciated Bryan Litz's contribution to ballistics and his books are well written and understandable by shooters like me. All my barrels, loads and bullets were based on SG = 1.4 and the stability factor may vary from 1.3 to 1.5 depending on the bullet I use.

SG = 1.4 was last years value and now Bryan thru testing has come up with 1.5 as being the preferred value, as it provides max BC as well as stability. All good stuff and a big thank you to Bryan!

My new barrels will be bought with twist rates that provide SG = 1.5 but so far I have not experienced any first hand stability problems shooting 1.3 to 1.5. Temperatures in Oz always have a plus sign in front of them for competitions and are mostly shot at not much above sea level.

Recently my shooting buddy has been developing a 7mm Dakota load for the new hyper high BC, Berger 195gr hybrid bullet. We both shoot F-Open and he wants this gun for long range work out to 1200 yards. The barrel is a 32" Krieger 5R, 8.5 twist rate, 1.25" parallel with a Kelby F-Class action.

He is experiencing bullet blow ups at 3020fps and we are wondering if the SG = 1.5 requirement of a 8.5 twist rate is responsible. At 3020fps the bullet is spinning at approx 255,800 rpm. Have Berger Bullets or anyone else performed tests on the mechanical integrity of bullets versus spin rate?

Dropping the velocity to 2900fps, (232,000 rpm) has reduced the bullet blow ups but still the occasional bullet just turns into a grey cloud on its way to the target. He has a identical barrel and chamber in 9 twist that will be tested sometime soon. It will be interesting to see if over revving the bullet is the cause. Maybe the 9 twist barrel will get the centrifugal forces back under control.

Can anyone contribute data to bullet rpm and mechanical breakdown? Could be interesting for a certain Berger Ballistician to workout the failure rpm of the bullet?

Ian
 
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Hard to believe a Krieger, especially the 5R, would be too rough, but I guess anything is possible. TDU: I would also like to see maximum rpm studies for bullets. Great idea. I heard Mr. Litz has been looking for new areas to study.
 
Ian,
Years back Berger changed their target bullets years over to thicker jackets for a reason… occasional blow-ups! Fast twists, high speeds, high round counts, fast shooting of long strings in overly long barrels and thin hunting jackets don’t go together.

Have a look at my warning here - http://forum.accurateshooter.com/threads/195g-berger-hybrid-284-caliber.3878807/page-3#post-36615236

Even in a 1-9” twist you might have problems in a 32” barrel at over 3k fps. I liked the bullet but will not use them in an FC match just in case I might lose one. I did that with 6.5/284 years ago. 30+” barrel, high round count, shooting fast, it happened the near the end of a 20 shot match:( Went to moly coating and that never happened again. You might try coating them FWIW. After the thick jackets came out I dropped the moly coating and shoot naked bullets now:)
 
Interesting!

The barrel is only approx 200 rounds old and work was done by a quality gunsmith. The barrel will be going back to said gunsmith for bore-scoping to make sure that there is no problem with it.

Larry, the last blow up was at the end of a 15 shot string so that corresponds to what you have commented on above. Wondered whether thin jackets could be a problem and it seems it could well be the issue, especially if coating the bullets made the problem disappear. What has a long barrel got to do with the blow-ups? Curious on the theory behind this simply because I would like to learn whats going on.

At 2900 fps this high BC bullet still offers many advantages, (ballistically) over most other options, if only the bullets could stay together. It seems one step forward and two steps backwards with this bullet for F-Open.

I shoot the 30 cal, 230, (2850) and 215gr hybrids, (2920) for pretty much the same BC, but with approx 20% more recoil. I think for the time being I wont be trying the 195gr bullets till I see what pans out with my buddies load development with them. He wants to shoot them in F-Class Match rifle comps at 1200 yards with 15 shot matches. From what I'm reading maybe he should be looking to go back to his 30 cal and the heavy bullets until the problems are worked out.

I'm sure Berger (or any manufacturer) could do some research and find out what happening to these bullets in relation to velocity and rpm and evaluate if these forces are responsible for the bullet blow ups. High speed video could show the state of the bullet leaving the barrel, etc, I'm sure data could be gathered to work out critical forces causing the phenomena.

Ian
 
Note that those bullets were developed for hunting and are not target bullets. Doesn't mean they're not great target bullets as well, but your thin jacketed target bullet premise isn't correct.

I'm building a 7/338 Lapua to shoot those bullets in ELR matches. When I was setting up the reamer, Kiff offered the 7/338 reamer drawing that EOL used for testing those bullets. If the group that sponsored the development of that bullet couldn't blow them up with a case capacity 20% higher and higher available pressures than that Dakota, another half inch of twist rate isn't likely your problem.
 
Larry,

Thank you for those links! It seems Berger Bullets have looked at what causes the bullet blowups in some detail.
Basically it comes down to heat in the bullet causing the lead core to soften. The list below is not in order of priority, just the points I gained from reading Eric Stecker's post on the forum.

1. Excessive rpm
2. Excessive barrel length
3. Thin jacketed bullets
4. Excessive overbore loads
5. Lack of barrel carbon buildup or lubrication
6. Poorly manufactured bullets
7. Number and type of grooves in the barrel

The heat is generated by barrel friction and load and the heavy jacketed bullets have a thicker base and tail section which help isolate some of the heat source from the inner lead core. It seems while increased rpm increases the centrifugal forces as well as friction, there is not yet any measurable data collected on this.

Interestingly thick jackets don't really strengthen the structural integrity of the bullet but help to isolate some of the heat away from critical areas!

Eric Stecker published the thread I refer to back in 2007 and I wonder how much more is known now?

Ian
 
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If the core was close to a temperature where lead and lead alloys would soften at the muzzle the high speed IR photos would show an even jacket temp. Those IR photos are quite clear where the heat affected areas are. Curious why there is a narrow radial ring of less heat all the way around smack dab in the middle of the bearing surface area. Cannelure would be much farther forward. Hrmmm......

The thermal conductivity of copper is 11 times that of lead and a little more than that of 416R/4130.
 
GrocMax, looking at the thermal imagery I wondered if the heat shown on the bullet bearing surface from contact with the lands is perhaps the weak point and high centrifugal force results in it splitting open at this point? Obviously we can draw incorrect conclusions until we have results from actual testing.

In the meantime the two 32" barrels, (8.5T & 9T) will be getting shortened to 30" and tested back to back to see if it makes any difference. The twist rates change the rpm and it will be interesting to see if SG=1.3 works any better than SG=1.47 at 2900 fps. Not worried about barrel length and can drop to 28" if necessary as there is plenty of space for increasing the load in the case.

The problems encountered with this bullet mean velocity will be kept to 2900 fps for testing and competition use. Dropping the velocity much lower defeats the purpose for LRFC at 1200 yards as the 30 cals offer similar BC without the bullets exploding.

Ian
 
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ThunderDownUnder said:
GrocMax, looking at the thermal imagery I wondered if the heat shown on the bullet bearing surface from contact with the lands is perhaps the weak point and high centrifugal force results in it splitting open at this point? Obviously we can draw incorrect conclusions until we have test results from actual testing.

In the meantime the two 32" barrels, (8.5T & 9T) will be getting shortened to 30" and tested back to back to see if it makes any difference. The twist rates change the rpm and it will be interesting to see if SG=1.3 works any better than SG=1.47 at 2900 fps. Not worried about barrel length and can drop to 28" if necessary as there is plenty of space for increasing the load in the case.

The problems encountered with this bullet mean velocity will be kept to 2900 fps for testing and competition use. Dropping the velocity much lower defeats the purpose for LRFC at 1200 yards as the 30 cals offer similar BC without the bullets exploding.

Ian
Click to expand...
Ian,
Lots of correct answers to your issue with the 7mm 195 EOL bullet . Let me add one more source of heat that affects the bullet that is particular to target shooting. The heat of the barrel due to string fire shooting. The rest that Eric mentioned in his article on this also come into play. When you look at the list culprits that cause bullet issues . All but the "thin jacket"( the 7mm 195 EOL is a hunting bullet and has a thinner jacket) and poorly manufactured bullets contribute to heat and friction that can cause bullet failure. We are looking to bring out a 7mm 195 EOL target bullet so this will probably fix your issue you're having now. Keep an eye on our website for the announcement.
Take care,
 
Thank you Phil for your info and contribution!
The 195gr bullet has a great BC and so a tremendous potential to change whats currently used in long range F-Class competitions. Obviously to be widely accepted and successful the bullet needs to reach the target in one piece. Looks like Berger Bullets are about to make that happen with the introduction of a 195gr EOL Target bullet and that is very good news.

Ian
 
A related situation that might help the conversation...

Its a long story, but the essence is this. I've had Berger 90vlds blow up enroute to the target during a match. I know why, but the SG part I had not thought about so I did some calculations. I tuned my 90vld load at 70-75oC at 900ft elevation and had them shooting well from a 1:7 twist, 30in 223Rem at 2835fps. Using Bergers stability guide, they are about 1.42 SG (close to what you describe). Went to a match in MT last summer which was about 3000 ft elevation. Morning was cool and they shot extremely well for me (match winner). By second match it was in the high 80s-mid 90s. They were shooting awesome until shot 17 of the 20 shot string. Shot a low 5 (or 6). About a 2-3 minute wait before shot 18 due to length of time finding hole. Shot 18 was a 10. Shot 19 another low shot (5 or 6). At the time I had no idea what happened. Puller thought it was a 308 hole.

A fellow shooter though I was blowing the bullet up and he was correct. I grabbed a backup load of 80vlds. By shot 6 or 7 they were also grey cloud 100 yds out.

Turned out to be glaze carbon in the throat of my barrel. It was shredding the jackets. Had 725-750 rounds on barrel. Though I was cleaning adequately. But was not. This was confirmed with a borescope and a through cleaning. After cleaning, accuracy returned, but had to adjust the load.

Because of your post, I calculated the SG for my load at that match. Because of the temperature and the elevation difference SG = 1.61 even without accounting for the few fps increase that was likely at 95oC. So here is another example that might get people wondering about SG 1.4 to 1.6 effect, but I'm pretty confident that it was carbon.

I'm suggesting that you also might be seeing the effect of a buildup of hard carbon in the throat of your barrel. I had the luxury of immediately trying a different bullet and had good advice to help me figure out my issue. I was glad because it was a rough 2 hours in the pits trying to figure out how I shot a 5 and a 6 during my string having only dropped 1 point up to shot 17. ;)

Hope this helps,

Drew
 
Drew, interesting what you had to say, thanks!

I too have shot the 90VLD's fast, (3140fps) in my long range setup using the 22BR and have won a club 1000 yard comp using it. In light wind the 90VLDs are unbelievably good. It became my young sons F-Class rifle of choice with little recoil and max accuracy. I used a slower twist rate than you and never had a problem with bullet blow ups but did drop the velocity back to 3040 fps eventually to stop copper build up.

I now think after reading the available info, that this thread has provided, that high bullet rpm is only one contributing factor. I also think that the 195gr bullet is not designed for high velocity use such as in F-Class comps. Its probably suited to shorter barrel hunting rifles that deliver mid range velocities below 2900 fps.

It seems Berger Bullets have the solution by bringing out a thick jacket version for target shooters. The thick jacket hopefully will mean SG=1.5 is not a problem, along with long barrels and assorted barrel groove count and type configurations. Anyhow, we will all soon know as it seems the new bullet is not too far away!

Ian
 
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I'm sure there are a number of factors which determine whether or not a bullet self-destructs on the way to the target. From the point of view of physics, consider the centripetal force (F) acting on a spinning bullet. I don't know how to draw equations in a forum post, but it's F=Mass times the angular velocity squared times the radius. You can, for practical purposes, consider the weight of your bullet as the mass and the RPM as the angular velocity and your caliber as the radius, if you don't mind trampling all over the correct unit values.

The point is that the force required to hold a bullet jacket together goes up if you use larger calibers because they not only weigh more but because they are larger in diameter (radiusX2). But the force varies with the square of the rotational speed, so a higher muzzle velocity in a faster twist barrel makes this force go WAY up. In short, it means that shooting larger caliber bullets at higher muzzle velocities in faster twist barrels offers a real structural challenge to the bullet manufacturers.

When you think about it, keeping in mind that these things are spinning at a quarter of a million times per minute, it's a wonder they don't ALL blow up.
 
Mozella said:
When you think about it, keeping in mind that these things are spinning at a quarter of a million times per minute, it's a wonder they don't ALL blow up.
Click to expand...

It is pretty daunting to think of the stresses, what else can you think of that will hold together at a quarter-million RPM?
 
Looking through posts in the Long Range Hunting Forum, it seems plenty of people are using this bullet at over 3000 fps, some over 3100 fps in their 7mm RUMs and such without bullet blow ups. The difference is they are only using 26" barrels and very short shot strings.
 
I think the shot strings are a huge culprit here. I know too many people having good luck with the 28 nosler and 195 surpassing 3000fps to suspect the forces alone.

Another consideration.
Some types of rifling are harder on jackets than others.

Sometimes these big bullets have a lot of inertia and really refuse the initiation of spin.

I think lead angle, rifling pattern, copper fouling, and even bullet jump which may cause the bullet to resist the initial spin can cause significant smearing or tearing of the jacket as it engraves the rifling.
 
Ian, I like seeing that the 90vld can survive some higher velocities. I've pushed then just above 2900 in the 223R, with excellent accuracy, but too hard on the brass. I agree with you and your son, what's not to like about low recoil and high accuracy. Glad to see the next generation F-Class getting started young.

Bushman, totally agree about the other factors that can effect the jacket. I have been using a 4-groove barrel and have been told that a 5R would be easier on the jackets. Planning on trying eventually, but I have three barrels that I need to shoot out first.....No only if Berger can get some 90vlds into production I can get to it.

Drew
 
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