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Bullet failure article

 
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Old 02-13-2007, 11:08 PM
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Bullet failure article

Found this over on BR Central in the 1000 yd forum. Interesting.

[ QUOTE ]
Bullet Failure Causes and Solutions Defined

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As many of you know we have been working on the bullet failure (blow up) situation for some time. I have been collecting data from numerous shooters over the last 2 ˝ years ranging from general observations to controlled experiments. At Berger, we have been working with folks at MIT and with other top minds in metallurgy and ballistics. What I have below is a report on what we have learned.

To briefly review, bullet failure is when the bullet does not hit the target anywhere near the expected impact location. (This is not about the unexpected 8 or the fifth shot out of a bug hole group). This result can be observed as a shot that is driven way off course but does make it to the ground, a shot that appears as a puff of smoke 30 yards or more from the muzzle, and everything in between. The shooter can experience bullet failure with several shots or with one shot out of a string.

The wide range of results and conditions has made it very challenging to sort out the true root cause. The information below is meant to bring the true root causes to the surface. I am not suggesting that these causes exist in every situation; however, they cover the vast majority of bullet failures.

The first two root causes are responsible for the most bullet failures:

Excessive RPM resulting from high velocity and a barrel that has a twist rate much faster than is needed for the bullet used. We are working on determining the general RPM limits for various bullets. This will be a long project, and the data we have now is not enough to publish RPM limits.

Solution: Use twist rates that are the same as or close to (faster) published recommendation. When shooting cartridges that produce higher than normal velocity (high capacity wildcats) consider using twist rates slower than those published since the published twist recommendations are based on velocities achieved by standard cartridges. (How much slower is based on the situation however it will usually be only 1” slower)

Friction that produces heat that exceeds the melting point of lead. This result is observed most often by the puff of “smoke” that will be within the first hundred yards from the muzzle. The “smoke” is in fact molten lead. The puff of molten lead does not always occur during this failure. A core that becomes even slightly plastic will not make it to the target properly.

You have heard me talk about a combination of conditions that produces a failure. I have believed this to be true for a long time but frankly, it has only been recently that we have begun to truly understand what is actually happening. Once we started looking at the possibility of the core melting, all the puzzling information from the various reports began to make sense. This is going to be a lengthy post focused on identifying root causes and their solutions so I will not go into all the various conditions and ranges in which these conditions exist that support these findings.

The report that our bullets would fail while Sierras would not was particularly puzzling. We have known for a while that making the jacket thicker does not make the jacket significantly stronger. As it turns out, we were looking at it from the wrong point of view. We had been looking at a thicker jacket as being a tougher jacket and this just isn’t true, however when you have a thicker jacket you are moving the lead away from the source of the heat (friction between the barrel and the bullet which is mostly in the area of the rifling, not the grooves). Bullets that have thicker jackets are actually thicker in the base and sidewalls near the base, which moves the lead further away from the heat. This increases the amount of friction that the thicker jacketed bullet can realize before the lead core gets hot enough to melt.

Since thicker jackets are difficult to make concentric, we have two solutions. The first is that we are going to work on making thicker jackets for our long-range bullets. This is going to take time, as we will not produce jackets that are greater than .0003 TIR in wall thickness variation. This is harder to do with thicker jackets. The bullets we make now shoot very well and there are several ways that this failure-creating friction can be avoided as it has been by many shooters. Avoiding this condition is the second solution.

Solution for avoiding failure-creating heat using our current bullets: The goal is not to slow the bullet down but rather reduce the heat created by the friction. There are several ways to do this. (Keep in mind that each condition is not absolute and in fact works with other conditions to create failure. Since failures occur occasionally when all the conditions work together to create excessive heat we know that it will not take much to insure that failures are avoided)

First, you can consider your barrel length. It has been found that barrels longer than 28” are capable of produce failure-creating heat. Remember that the bullet is hottest at the muzzle. The more metal the bullet has to travel over, the hotter it gets.

Second, consider using moly, Danzac (tungsten disulfide), or any other dry lubricant as these reduce friction thereby reducing heat. I know moly is a hot button for many shooters however setting all other things aside it works great as a friction (heat) reducer.

Third, consider running a patch with a light amount of Kroil through your barrel prior to shooting. This will lubricate the barrel long enough until the carbon builds up enough to serve the same purpose. The first few shots will be erratic, but failure-creating heat is avoided. Barrels that are squeaky clean produce significant levels of friction if no friction reducers are present before firing. (I am not suggesting that you do not clean your barrel completely but rather pointing out how to avoid failure-creating heat when you start shooting).

Fourth, consider the bore diameter of your barrel (land height not groove depth). Some barrel makers can provide you with different bore diameters. Consider diameters on the larger side of the available options.

Fifth, consider the land configuration in your barrel. Six groove, cut, squared off rifling produces greater friction than a 5C or 5R type barrel. The 5C or 5R type rifling produces more friction than a three-groove barrel. I am not suggesting one is better than another; however, the friction generated by the different rifling designs should not be ignored.

Sixth, consider the cartridge you are using. Cartridges such as the 6X284 or any overbore wildcat are notorious for high velocity and barrel life consumption (rapid erosion). These are some of the main ingredients in failure-creating heat generation.

Please remember that the combination of components used in your rifle is a compromise. I have learned why Berger Bullets fail when others do not. I have decided to share this with you because I am committed to enhancing the experience for the shooter and in my opinion, more information is better even if on the surface this information makes us look bad.

Many shooters avoid failure-creating heat when they use Berger and find that Bergers work best for them. You can look at the information above as a reason not to shoot Berger or you can look at it as detailed instructions on how to make Bergers work for you without the concern of producing failures. If you value the accuracy that Bergers produce, then the above information details areas where you can make an easy compromise now that you have all the facts. If you value the conditions listed above that produce failure-creating heat more than you value the accuracy of Berger Bullets, then your decision is also made easier with these details.

The following root causes are responsible for bullet failures but in smaller numbers:

Human error that produces failure-causing condition. Let’s all admit up front that none of us are perfect. A shooter can create failure-causing condition in the barrel by improper break in, cleaning or storage (crown damage). Failure causing conditions can be created with the load as well. Using the wrong powder, not chamfering necks, excessively tight neck tension damaging the base of the bullet, poor handling practices can also lead to failures. Careful and appropriate firearm handling and loading practices usually avoid these failure causing conditions.

Tight or rough bore that actually tears jacket material away from the bullet. This is an extreme and rare condition that is easily identifiable with a bore scope, by slugging the barrel or by feeling for a spot that is more resistant to cleaning with a tight patch. Barrel makers are quick to resolve this situation.

Poor bullet fabrication such as too low or too high seating pressure. Low seating pressure can create a poor mechanical bond and/or air pockets that further destabilize the bullet. Seating pressure that is too high effects the copper jacket by producing a weakness where the nose can separate from the body. These conditions can be most easily detected by weighing your bullets, as too low or too high seating pressure is mostly the result of an extreme change in the mass of the lead.

Other examples of poor fabrication are any excessive lube on the cores (many bullet makers do not clean their cores before bullets are swaged) or debris between the jacket and the core can produce a weak bond, air pockets and/or significant stability issues through poor balance around the axis. Another poor fabrication condition that is easiest to avoid is lead that contains debris or significant air pockets due to double extrusion. This condition does not exist when a quality source of lead is used. Quality bullet manufacturers of which there are many can avoid all of these fabrication conditions.

There might be some other causes of bullet failures beyond those listed above but they happen is such rare occurrences that they have not been identified and should not weigh heavily on your mind.

It is my sincere hope that you find this information useful toward enhancing your shooting experience.

Regards,
Eric Stecker
Berger Bullets


[/ QUOTE ]

I'd tend to disagree w/their assesment on groove #s and the effects they have. How about yall [img]/ubbthreads/images/graemlins/confused.gif[/img]???

Here's the link in case that is a little too tough to read for you old codgers [img]/ubbthreads/images/graemlins/grin.gif[/img].

http://www.benchrest.com/forums/showthread.php?t=40028
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  #2  
Old 02-14-2007, 05:25 AM
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Posts: 920
Re: Bullet failure article

[ QUOTE ]
I'd tend to disagree w/their assessment on groove #s and the effects they have. How about yall [img]/ubbthreads/images/graemlins/confused.gif[/img]???

Here's the link in case that is a little too tough to read for you old codgers [img]/ubbthreads/images/graemlins/grin.gif[/img].

http://www.benchrest.com/forums/showthread.php?t=40028

[/ QUOTE ]

Yup... me too.

Plus I disagree with their theory about barrel friction/lead melting.

If you consider the amount of time in the barrel (about 1.5/1000ths of a second), the thermal coefficient of jacket metal is not fast enough to melt the lead before the bullet is in free air.

Plus, given long barrels of the same length (I shoot a lot of 28" tubes), if a bullet is coming apart in a 10" twist barrel, it won't in a 12" barrel... and the friction/length/time in barrel" is the same.

Plus... if you are shooting a short barrel (20"), with a twist/velocity ratio that gives you the same spin, you find that the bullets still come apart, with 1/3 LESS friction.

Hmmmmmm??

I have been chasing this issue for about 25-30 years, and as I see it, it is simply a matter of jacket strength. My first blow ups were back in the 1970's a .220 Wilson Arrow, using 50gr and 55gr Hornady SX's that had 0.009" jackets.
They blew up at around 3,400 to 3,500fps.

The same design (50gr SP's) with 0.18" jackets could be driven out of the .220 Wilson Arrow at the speed of light without failure. Same barrel time, same velocity, same friction (actually more friction, because the 0.018" jacket had more resistance to rifling deformation... therefore more friction).

It is pretty well known that J4 jacket material is softer than the jackets made by Sierra and the others.

Berger is in a tough spot on this. They are now owned by J4 (IIRC), so they can't switch jackets, so they can't solve this problem, and the 1000yd bench shooters are bitching about the Bergers blowing up on the way to the target...

The ol' "Rock and a hard place"

.
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Old 02-14-2007, 08:19 AM
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Eyes Wide Open............

Recently, I was shooting a 7 STW with the 140's going 3650 out of a long barrel 3 Groove Pac Nor barrel. My back stop was a large round hay bale. This rifle is extremely accurate to say the least and I only use a small target to shoot at. The hay bale is only 6 or so inches behind the target. After a couple of shots, I noticed that the hay was smoking, my bullets were impacting the top edge of the target. The hay was actually being set on fire by the hot bullets! Since I was using a 8-32 scope, it was clear to see, shot after shot!

Years ago, I was friends with a couple of guys Steve Chernicky and Gene Harwood. Steve built all of the Olympic 22 rimfires that beat the tar out of Russia, Gene was one of the head chemists with Shell Motor Oil had more than 100 sets of custom bullet making dies. They were involved with Bill Davis of Arbedene Proving Grounds in the development of a new Rim Fire ammo that was shot in the Olympics that we now know as Federal Gold Metal Match 22 Rim fire.

They also worked on developing new bullets for the Cross Country Ski event in the Olympics. There were many developments in gun industry that we now enjoy.

One of the things that Bill Davis found out is that the base of a certain brand of bullets deform on the long VLD boat tail bullets due to rotational stresses. They took pictures of the 105g 6mm as it exited the barrel. It all came down to alloy content of the core & jacket thickness.

Steve had a batch of lead wire made up at the foundry that had 5% tin in it and it eliminated the entire mess of boat tails collapsing. I got a bunch of the lead wire and loved the way that it improved the accuracy on my 70g boat tail bullets shot at 4000+ fps.

There is another issue that no one ever speaks of and that is that the bullet heats up from friction with the air, which is what was happening with my 140g bullets shot out of my 7 STW. There is a huge difference in performance on big game between the standard 140g NOsler Ballistic tip and the 140g Nosler Winchester Combined Technology bullet due to the coating on the CT bullet. The coating on the CT bullet helps reduce friction with the air, thus the core is harder when it impacts the animal. The 140g CT bullet works every shot on deer while the std 140's do not. Not to mention that you can shoot the CT bullet much faster due to the decreased friction in the barrel, it simply does not build the pressure that the naked bullet builds allowing you to shoot them much faster!

In sumary, some of the 6 mm long VLD boat tail bullets of a certain brand collapse that are made from J-4 jackets due to rotational stresses above 3050 fps, it does not seem to to happen to Sierra bullets due to thicker jackets and higher antimony content.

Bullets do indeed heat up in flight due to friction with the air, but there has to be a relationship with speed.
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Old 02-14-2007, 10:44 AM
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Re: Eyes Wide Open............

[ QUOTE ]
alloy content of the core & jacket thickness.

[/ QUOTE ]

[ QUOTE ]
thicker jackets and higher antimony content

[/ QUOTE ]

Now then there is some interesting information.

I bet it is more difficult to make an accurate bullet with harder cores and thicker and harder jackets.

I doubt that the core "melts". I suspect that it gets soft and slips inside the jacket plus loses tensile strength, deforms and the rotational spin goes bad and rips it apart at the rifling creases.
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Old 02-14-2007, 10:47 AM
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Re: Eyes Wide Open............

[ QUOTE ]
[ QUOTE ]
alloy content of the core & jacket thickness.

[/ QUOTE ]

[ QUOTE ]
thicker jackets and higher antimony content

[/ QUOTE ]

Now then there is some interesting information.

I bet it is more difficult to make an accurate bullet with harder cores and thicker and harder jackets.

I doubt that the core "melts". I suspect that it gets soft and slips inside the jacket plus loses tensile strength, deforms and the rotational spin goes bad and rips it apart at the rifling creases.

[/ QUOTE ]

That's what I think too...

.
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Old 02-14-2007, 11:02 AM
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Re: Bullet failure article

[ QUOTE ]
The first two root causes are responsible for the most bullet failures:

Excessive RPM resulting from high velocity and a barrel that has a twist rate much faster than is needed for the bullet used. We are working on determining the general RPM limits for various bullets.

[/ QUOTE ]

That is very true and simple science. When a bullet is spinning at an excessive RPM, it is cytrifical force that cases the jacket to tear apart upon impact.

While the above statment is true, this statment is a theory (at least to me). It is logical to conclude that the depth and quantity of the rifles's land's engravings on that jacket would come into play to some degree. Obviously, if the bullet was at a normal RPM and the barrel that fired it was in normal specifications, the bullet's jacket wouldnt "fail". I am sure there is some truth to the article. Some being the operative word. Note that one of the keys here to this article is excessive RPM. Take that key out of the picture and the article wouldnt make any sense.
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