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Comet tails

There are some nuggets of information in Kevin's post. Thin jackets can be more accurate but they present a problem with long shot strings. Example Berger 338 300 gr. The first lot was the most accurate 338 bullet any of us had seen in 1K BR. The problem is when the LR hunting crowd pushed them to 3000 FPS they started losing bullets. Solution- thicken up the jacket. The phone stopped ring at Berger but the bullets weren't as accurate.
If you examined a bullet ogive closely it is a series of folds. Depending on the metallurgy of the base metal, hardness from drawing the cup etc. you can get fractures on the fold lines.
 
That’s th conundrum, exactly. It’s far easier to draw very thin jackets and keep them very concentric. Thicker jackets are much, much more difficult to draw uniformly, and concentricity suffers accordingly. You’ll virtually never see these bullets blow up in flight, but they shoot like crap in about 99% of the cases. This is precisely why all the bullet makers that I’ve ever been involved with utilize very thin jackets on their match bullets. It’s the typical trade-off; getting them thin enough to draw well, while not quit crossing th line where they become frangible in flight. Fine line, and it’ll move on you without warning sometimes.
 
KevinThomas said:
That’s th conundrum, exactly. It’s far easier to draw very thin jackets and keep them very concentric. Thicker jackets are much, much more difficult to draw uniformly, and concentricity suffers accordingly. You’ll virtually never see these bullets blow up in flight, but they shoot like crap in about 99% of the cases. This is precisely why all the bullet makers that I’ve ever been involved with utilize very thin jackets on their match bullets. It’s the typical trade-off; getting them thin enough to draw well, while not quit crossing th line where they become frangible in flight. Fine line, and it’ll move on you without warning sometimes.
Click to expand...

Rule #1 around here is don't do anything to make the phone ring.
Rule #2 build the most accurate rifle possible while referring to rule #1
 
Two targets 50yds top is ppu 139gr FMJ bottom is Geco 156gr RNLP. No Comet tails on the Geco but slight ones on the PPU. Very Strange? I am assuming Velocity is the culprit here. I am going to do some abslute min loads. With RL-22 and H4350, to see if it’s poweder bullet combination coupled with velocity.
 

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So if I understand the phenomena correctly as explained in earlier posts by Kurt and Dave, heat generated during the engravement process and from friction as the bullet traverses the bore leads to localized melting of the surface of the lead core underneath the region of land engravement, which is then expelled via outgassing through small holes or cracks in the jacket, ultimately resulting in the lead deposits on the target that appear as "comets". In this scenario, it would still seem that as opposed solely to heat, a big part of the problem is still the thickness of the jacket, as Kurt pointed out, because loss of "containment" of the lead core through a small hole or crack is likely the beginning of the end. In other words, as long as the jacket integrity remains intact, it doesn't seem as though melting of the lead core surface would be as much of a problem.

In my mind, this begs the question of exactly what is the minimum land height necessary for a given caliber to functionally engage and engrave the jacket and impart the necessary spin to a bullet? Stated in a different way, are the lands in typical barrel bore/groove configurations taller than they absolutely have to be to work properly? I have .308s with both .299"/308" and .300"/308" land/groove geometry, and .298"/.308" barrels are not uncommon. So it seems as though there is some amount of latitude in bore/groove dimensions, at least for certain calibers.

In my hands and those of quite a few other F-TR shooters here at Accurate Shooter, the issue with jacket failures has been observed predominantly in .223 Rem F-TR rifles loading heavy .224" bullets such as Berger's 90 VLD, and Sierra's 90 SMK. In talking to a number of other people that have experienced this issue, it does appear to be exacerbated by using a twist rate faster than 7.0, but that is not the sole cause as people using 7.0-twist barrels have also experienced blow ups. All my .224" barrels are 5R, 0.218"/0.224" land/groove configurations. So my question is, could I get away with using a 0.219"/0.224" barrel, or even a 0.220"/0.224 or 0.221"/0.224" configuration? I'm just curious as to whether a modest decrease in land height might be enough to help minimize the potential for jacket failure without sacrificing precision, if the failure is caused in part due to the relative "thinness" of the jacket itself with a particular bullet.
 
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Ned Ludd said:
So if I understand the phenomena correctly as explained in earlier posts by Kurt and Dave, heat generated during the engravement process and from friction as the bullet traverses the bore leads to localized melting of the surface of the lead core underneath the region of land engravement, which is then expelled via outgassing through small holes or cracks in the jacket, ultimately resulting in the lead deposits on the target that appear as "comets". In this scenario, it would still seem that as opposed solely to heat, a big part of the problem is still the thickness of the jacket, as Kurt pointed out, because loss of "containment" of the lead core through a small hole or crack is likely the beginning of the end. In other words, as long as the jacket integrity remains intact, it doesn't seem as though melting of the lead core surface would be as much of a problem.

In my mind, this begs the question of exactly what is the minimum land height necessary for a given caliber to functionally engage and engrave the jacket and impart the necessary spin to a bullet? Stated in a different way, are the lands in typical barrel bore/groove configurations taller than they absolutely have to be to work properly? I have .308s with both .299"/308" and .300"/308" land/groove geometry, and .298"/.308" barrels are not uncommon. So it seems as though there is some amount of latitude in bore/groove dimensions, at least for certain calibers.

In my hands and those of quite a few other F-TR shooters here at Accurate Shooter, the issue with jacket failures has been observed predominantly in .223 Rem F-TR rifles loading heavy .224" bullets such as Berger's 90 VLD, and Sierra's 90 SMK. In talking to a number of other people that have experienced this issue, it does appear to be exacerbated by using a twist rate faster than 7.0, but that is not the sole cause as people using 7.0-twist barrels have also experienced blow ups. All my .224" barrels are 5R, 0.218"/0.224" land/groove configurations. So my question is, could I get away with using a 0.219"/0.224" barrel, or even a 0.220"/0.224 or 0.221"/0.224" configuration? I'm just curious as to whether a modest decrease in land height might be enough to help minimize the potential for jacket failure without sacrificing precision, if the failure is caused in part due to the relative "thinness" of the jacket itself with a particular bullet.
Click to expand...

I think a good start would be a slight increase in bore diameters but the extreme twist rate has to be factored in also. I'm working on a project that does just that but I not at liberty to discuss it. From that project one problem is finding a barrel maker with the tooling for larger bores. The extremes never seem to work in a consistent and predictable manner.
 
In 2008 I was involved in some bullet testing of the new at the time Berger 140 “thick jackets”. I received a bucket of bullets from the first run just to verify the accuracy vs the original “thin jackets”.

Mid Tompkins was also sent bullets to test the reliability of the new jackets at 1000 yards in Phoenix. He and some other people compared both in a rough barrel that was known to “blow up” some thin jackets. Needless to say the thick jackets had no problems but I asked Mid how far down range the thin jacket bullets flew before the “blew up”. He said they could see grey vapor trails start at any distance from right out of the muzzle to several hundred yards down range. He showed me a bullet that landed 700 yards down range. I wish I had a pic of it. It was sliced open at one of the land marks maybe half the length of the bearing surface and a small section of jacket peeled back like an open door. A glob of lead was hanging out of the side of the opening. Obviously molten lead came out of the opening, the bullet became unstable and just fell out of the sky landing on top of the 300 yard berm.

At longer range I believe the OPs bullets will do about the same thing. Shed lead, become unstable and fall out of the sky.

Jeff
 
Dave - thanks for your reply. I spoke with Bartlein earlier today, and they do make 0.219"/0.224" bore/groove configurations. Whether half a thousandth less land height on either side would be enough to make an appreciable difference in this specific case for a given twist rate remains the question, but I guess it certainly might not hurt if the precision was comparable.
 
DaveTooley said:
I think a good start would be a slight increase in bore diameters but the extreme twist rate has to be factored in also. I'm working on a project that does just that but I not at liberty to discuss it. From that project one problem is finding a barrel maker with the tooling for larger bores. The extremes never seem to work in a consistent and predictable manner.
Click to expand...
Dave,
Any experience with Marlin's Microgroove rifling? According to the patent they are only cutting the grooves .0015" which would make a much larger bore, .304" for a .308" barrel. I would think in high intensity cartridges that throat erosion would be rather quick though.
 
Ned Ludd said:
So if I understand the phenomena correctly as explained in earlier posts by Kurt and Dave, heat generated during the engravement process and from friction as the bullet traverses the bore leads to localized melting of the surface of the lead core underneath the region of land engravement, which is then expelled via outgassing through small holes or cracks in the jacket, ultimately resulting in the lead deposits on the target that appear as "comets". In this scenario, it would still seem that as opposed solely to heat, a big part of the problem is still the thickness of the jacket, as Kurt pointed out, because loss of "containment" of the lead core through a small hole or crack is likely the beginning of the end. In other words, as long as the jacket integrity remains intact, it doesn't seem as though melting of the lead core surface would be as much of a problem.

In my mind, this begs the question of exactly what is the minimum land height necessary for a given caliber to functionally engage and engrave the jacket and impart the necessary spin to a bullet? Stated in a different way, are the lands in typical barrel bore/groove configurations taller than they absolutely have to be to work properly? I have .308s with both .299"/308" and .300"/308" land/groove geometry, and .298"/.308" barrels are not uncommon. So it seems as though there is some amount of latitude in bore/groove dimensions, at least for certain calibers.

In my hands and those of quite a few other F-TR shooters here at Accurate Shooter, the issue with jacket failures has been observed predominantly in .223 Rem F-TR rifles loading heavy .224" bullets such as Berger's 90 VLD, and Sierra's 90 SMK. In talking to a number of other people that have experienced this issue, it does appear to be exacerbated by using a twist rate faster than 7.0, but that is not the sole cause as people using 7.0-twist barrels have also experienced blow ups. All my .224" barrels are 5R, 0.218"/0.224" land/groove configurations. So my question is, could I get away with using a 0.219"/0.224" barrel, or even a 0.220"/0.224 or 0.221"/0.224" configuration? I'm just curious as to whether a modest decrease in land height might be enough to help minimize the potential for jacket failure without sacrificing precision, if the failure is caused in part due to the relative "thinness" of the jacket itself with a particular bullet.
Click to expand...
Ned, As to your question about the minimum land height necessary to engage the bullet I believe it is much smaller than most people think. For example the .17 barrel that I've offered as an example would accept a .169 gauge pin in the bore when I chambered it. So .172 - .169 = .003 total or .0015 per side. A land height of .0015 is slim by any standard. Of course a land height like that comes at a cost. There's not much steel there and erosion is seen at a very rapid pace. It surprised me that it lasted as long as it did. I was not kind to it in the p-dog patch.
 
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Dave, those pictures are awesome. Probably not so awesome at the time you were fighting the problem but they are pretty cool to see now.
 
STS said:
Dave, those pictures are awesome. Probably not so awesome at the time you were fighting the problem but they are pretty cool to see now.
Click to expand...
Someday I may revisit that project. It sure had potential. I still have the bullet dies and a bunch of jackets.
 
If the source of the leak is due to a stress concentration left by the corner of the rifling, it might make sense to use more grooves and to put a radius on the corners of the lands. Or maybe vary the ratio of land area to groove area. I bet there is an optimal configuration and we may not be using it.

And this may be nitpicky, but are we really taking gas when we say offgassing or more of an aerosol of suspended liquid lead (which I would imagine would solidify almost instantly)?
 
damoncali said:
If the source of the leak is due to a stress concentration left by the corner of the rifling, it might make sense to use more grooves and to put a radius on the corners of the lands. Or maybe vary the ratio of land area to groove area. I bet there is an optimal configuration and we may not be using it.

And this may be nitpicky, but are we really taking gas when we say offgassing or more of an aerosol of suspended liquid lead (which I would imagine would solidify almost instantly)?
Click to expand...

If I had the money and time here's what I would do. At least to start with.
Pick several problematic caliber bullet combinations which require a twist rate faster than 1-9 to achieve an Sg of 1.5 and higher.

Phase 1
1. use cut rifled barrels to maintain dimensional uniformity from barrel to barrel
2. increase bore diameter by .004" leaving land height of .002"
3. odd number of lands to reduce jacket stress. Test 3,5 and possibly 7 grooves
4. land configuration canted
5. vary total land width as percentage of the circumference. Probably three different widths would
provide data needed.
6. test test test

Phase 2 assuming some success in phase 1

1. increase twist rates to determine the point where RPM's affect bullet integrity.
2. increase case capacity to increase velocity/RPM's, find and define failure points for each combination. That's assuming you get this far.
 
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