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Bullets vaporizing in mid-air

 
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  #1  
Old 06-16-2009, 03:00 PM
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Bullets vaporizing in mid-air

I have been having a perplexing problem with my 6mm-284. the barrel is a pac-nor 7 twist.
The problem is that the bullets are coming apart in midair about 50 to 100 yards from the muzzle. however not all the bullets come apart. it seems that after the barrel warms up with 10 or so rounds thru it that the bullets start to come apart. barrel only has 250 rounds thru it but was doing this from new. I had a smith inspect it with a borescope and everything looked good. the first bullets i used were berger hunting 115's then 105 amax then 105 wildcats and now i am having better luck with 107 MatchKings. also some berger 115 match are on the way to try.

I had the bergers loaded down to 2800fps and still had the problem. The 107MK are at 3100fps and i am still loosing 15% or so.

I emailed Pac-nor and they said that conventional bullets will come apart between 280,000 and 300,000 RPM. Yet i hear of people using 6 and 7 twist barrels that are not having any problems.

Is there something that i am missing here?
any help would be appretiated.

Tye
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  #2  
Old 06-16-2009, 03:19 PM
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Re: Bullets vaporizing in mid-air

Howdy Tye,

Not exactly unheard of, and not too uncommon in some combinations. Berger had some problems with this some time back and have since modified their line to eliminate such problems. You've already touched on part of the answer here; heat. Eric Stecker of Berger Bullets did some very extenisive testing with MIT and high-speed thermal images of bullets in flight to determine what was causing this phenomenon. They took a serious look at the causes here, and spent the time, effort and money to do it right. It comes down to heat, migrating through the jacket and melting the core, or at least a boundary layer of it under the jacket. Once the core goes plastic or liquid, any void in the jacket will allow it to outgas, and the bullet blows. By reviewing the thermal images, Eric was able to discern the actual temperatures of various areas of the bullet in flight, and it's not what you'd automatically expect. The base, for example, stayed quite cool despite being in contact with the hot powder gases. The ogive (not surprisingly) didn't heat up too badly. The real hot spots were the meplat, due to the friction of the air and nose pressure. The worst, though, was the bearing surface, particularly where the rifling was engraved upon firing. This portion hit several hundred degrees, almost instantly. The analysis concluded that the jackets were thin enough to allow the heat to penetrate to the core, and bad things happened. To resolve this, they thickened the jackets to allow for better "insulation" of the core, and the problem now seems to be entirely resolved. Just look for the bullets that they designate as "Target" designs, and this should take care of it. Having described the causes here, you can see that very long bearing surface bullets are a potential sore spot for this condition. High velocity and pressures can aggrivate this, but they alone are not totally to blame. One of the worst examples I've ever seen on this occurred (repeatedly and very reproduceably) in a 6.5x55 Swedish Mauser. The outgassing in that combination was so pronounced that the comet tails actually cut the target at 100 yards. Despite this, few of those bullets actually blew up, at least at that distance. A hotter day, a tighter bore, a slightly larger diameter bullet and who knows. Just one of the wierd things you'll see, but yeah, they're out there.

Hope this helps, even if only to explain what's going on.

Kevin Thomas
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  #3  
Old 06-16-2009, 03:39 PM
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Re: Bullets vaporizing in mid-air

Kevin,
Thanks that is the best explination that I have heard. it all makes sense now!
it is very frustrating especially when you hear of other people not having any issues and running 1 in 6 twists.

not sure if this may add to the problem but it is a 6 groove barrel. would a 4 groove be easier on the bullets?

Tye
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Old 06-16-2009, 03:54 PM
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Re: Bullets vaporizing in mid-air

Glad to help! As I'd said, there's some issues involved that most folks overlook. It's pretty natural to assume that it's strictly a velocity/pressure thing and let it go at that. The encounter that I described with the 6.5x55 pretty well trashed that notion, and that was with another maker's bullet, one that wasn't known for this sort of problem. So there was something else. eric finally nailed this all down, and kudos to him and the Berger crew for sharing the results. from this, yeah, I'd guess there could very well be some difference in the 6 vs 4 grove barrels. Which would be better, I couldn't say. You might discuss this with a barrel maker (it's referred to as aspect ratio by most of them) as to which is likely to cause more friction on the jacket during its passage through the bore. In the meantime, try the "Target" bullets from Berger, and that could very well solve the entire issue. I notice that you were using the "Hunting" version in your original post; these are their original design, with the old-style thin jacket.

Whose barrels are you thinking of, anyway?

Kevin
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  #5  
Old 06-16-2009, 04:08 PM
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Re: Bullets vaporizing in mid-air

Kevin,
I have been looking at the shilen rachet rifling a little bit lately. but have not come to any conclusions.
I was hoping someone might have had experience with the differences in rifling geometry.

Tye
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  #6  
Old 06-16-2009, 04:52 PM
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Re: Bullets vaporizing in mid-air

Tye,
Shilen makes a good barrel, but then again, all of the top manufacturers do. I've got several friends who use their ratchet rifling in barrels for silhouette competition. One of them is a two-time world Silhouette champ and pretty finicky about his equipment. That may not be directly related to the issue of aspect ratio, but I have no question about the barrel's being good.

Talk to the makers themselves. They're going to do their very best to steer you into a barrel that won't give you any problems. Be clear about what the issue/concern is here, and they'll set you up properly. This business runs on reputations, and these folks are very protective once they've built a good one.

Kevin
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  #7  
Old 06-16-2009, 06:14 PM
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Re: Bullets vaporizing in mid-air

A 1-7 twist is VERY fast for a 6mm-284. 1-8 is about as fast as I recommend even for the 115 gr Bergers and if you want to shoot the 105 gr Bergers or A-Max, the conventional 1-8 twist will be very hard on them and you will see either bullet failure in flight or extreme accuracy lose at velocities over 3200 fps, sometimes less.

For those that want to use the 105 gr Bergers or A-Max bullets, I recommend to my customers we use a 1-9 twist barrel for their rifles and with the velocity potential of the 6-284, these work great.

The 107 gr SMK will handle much higher RPM levels as well as velocity levels because their jackets are very thick compared to the Berger, A-Max or Wildcat bullets.

With your 1-7 twist, I would be suprised if you find any bullet other then the 107 gr SMK that will allow you to load anywhere near the velocity potential of that chambering. I would suspect even the 115 gr Bergers may have issues but they may drop your velocity ceiling enough that they may be usible.

Good Luck.
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