Bullets vaporizing in Mid-Air

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sandytye

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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?:bangHead::runaway:
any help would be appretiated.
 
It's probably a combination of fast twist and higher then average speed which increases the RPMs, throw in a new barrel and that is possible. It may start behaving once your barrel is broken-in.
 
Here are velocity/RPM for a 7" twist barrel

2000 206,000
2500 258,000
2600 268,000
2700 278,000
2800 288,000
2900 299,000
3000 309,000
3100 319,000
3200 330,000
3300 340,000
3400 350,000
3500 360,000

So standard bullets will start coming apart above 2700 fps (as you found out). It is also a new barrel with sharp rifling, which will "cut" the jackets better, creating a stress point on the jacket.

Also when compared to a .22" bullet, the slightly larger diameter of the 6mm will also create greater centripetal forces wanting to tear the bullet apart.
 
I think I will be trying some lapua 105's and some 115 DTAC's. as well I will be giving some WS2 a try in an effort to reduce the friction.
I also found out that pac-nor barrels are 0.236" bore diameter which does little to help the heat generated.

Thanks for the ideas.
 
There is an excellent post in the precision rifles forum on light bullets with fast twists. Here's part of it.

I got this explination on another forum as I am having the same problem in my 6mm.

"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.""

it seems that there is a lot of heat on the meplat so the plastic tip may help by reducing the surface area and thus the extra heat generated.
Click to expand...
http://www.canadiangunnutz.com/forum/showthread.php?t=360016
 
I had a custom triple deuce with a Gaillard 1:7 that would blow up Hornady SXs or Sierra Blitzs, but when loaded with regular jacketed soft points or with match bullets and it was never a problem.

You know it happens when your target doesn't get hit and the shot felt good. Some folks report seeing dust in the air at mid range, but I never have.

The problem is solved by choosing a bullet with a heavier jacket and by dropping your velocity. Choosing the heavier 115 gr bullets should solve your problem.
 
When there are a number of people on the firing line and someone reports a miss, "but the shot felt good" , it generally attracts the attention of other shooters who will watch the shooter and downrange target. When the bullet disintegrates in air it will be spotted usually 75-100yds. down as a cloud of dust no target impact at 200. I have seen it numerous times from a freinds 7mm mag. using 162 A-max bullets, this was a 1-10 twist barrel, 3100 fps. , it had I believe over 2000 rds through it at the time aand was so eroded to get the bullets to the lands they were out of the case completely.
 
shotgunjoe said:
When there are a number of people on the firing line and someone reports a miss, "but the shot felt good" , it generally attracts the attention of other shooters who will watch the shooter and downrange target. When the bullet disintegrates in air it will be spotted usually 75-100yds. down as a cloud of dust no target impact at 200. I have seen it numerous times from a freinds 7mm mag. using 162 A-max bullets, this was a 1-10 twist barrel, 3100 fps. , it had I believe over 2000 rds through it at the time aand was so eroded to get the bullets to the lands they were out of the case completely.
Click to expand...

First time you see it is neat though isn't it? Especially on a humid day. Almost looks like flak.
 
I have blown up a goodly number of bullets in the 220 Swift over the years, and soon found out which bullets were up to the job, and which were not. The blowup appears as a grey "dust" cloud out in front, anywhere from 25 yards to around 75 yards from the muzzle [obviously no hit on the target] I once shot at a crow in a tree and had a small piece of the jacket land on the hood of my IHC Scout when the bullet blew up out front. Crow flew away, unscathed, LOL.
Interesting phenomenon, but reasonably common. Regards, Eagleye.
 
Eagleye said:
I have blown up a goodly number of bullets in the 220 Swift over the years, and soon found out which bullets were up to the job, and which were not. The blowup appears as a grey "dust" cloud out in front, anywhere from 25 yards to around 75 yards from the muzzle [obviously no hit on the target] I once shot at a crow in a tree and had a small piece of the jacket land on the hood of my IHC Scout when the bullet blew up out front. Crow flew away, unscathed, LOL.
Interesting phenomenon, but reasonably common. Regards, Eagleye.
Click to expand...

All the crows I have shot with 420 grain cast bullets in a Marlin 45-70 never flew away. I guess the 45-70 just doesn't have the velocity to make bullets come apart.:D The critters came apart though, I am happy with that.:)
 
303carbine said:
All the crows I have shot with 420 grain cast bullets in a Marlin 45-70 never flew away. I guess the 45-70 just doesn't have the velocity to make bullets come apart.:D The critters came apart though, I am happy with that.:)
Click to expand...

303; I can only imagine what a crow looks like after a hit with that 420 grainer!! :eek: However, I believe that I might have an advantage with my Swift at 400+ yards, and I still get similar results. I have had many hits where it seemed that all that hit the ground were the wings....the rest just floated away in the breeze! :D;) Eagleye.
 
Eagleye said:
303; I can only imagine what a crow looks like after a hit with that 420 grainer!! :eek: However, I believe that I might have an advantage with my Swift at 400+ yards, and I still get similar results. I have had many hits where it seemed that all that hit the ground were the wings....the rest just floated away in the breeze! :D;) Eagleye.
Click to expand...



The 100 yard birds got launched backwards when shot with the big cast slugs. Ususally they ended up in two pieces several feet apart....:shotgun:
I can't seem to get them to vaporize like they do with a 22-250.:eek:
 
6-284 twist rate

For this caliber a twist rate of 1in8 is the norm. You may never find the 1in7 will do the job. If you find a bullet that survives the fast spin, it may not be the most accurate bullet you could have selected if the twist was 1in8. If you're competing with this, you've got a problem.
Bill
Eagleye, leave the crows, at home here they drive off the bigger offenders Eagles and hawks after my mousers!!
 
Yes it is neat the first time you see it....and the second.....and third...etc,etc especially when you are shooting a provincial match and it happens to someone else in your class,all fired rounds count:D:D:D:D:D
 
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