comet tails

C

charlie AR15

Guest
Guys, this is a new one for me. I have a gun I'm working up loads for and set the target at 25 yds. for aiming purposes only so I could use my chrony. I noticed I was getting a comet tail on the bullet holes; but when I went up 1/2 a grain of powder, one shot went through sideways ! I know there's a simple answer as to why and how to solve the problem; but I've got a case of CRS and can't remember what it is ! Can anyone help me? I'm not much of a benchrester but knew you were and may be able to help. What I'm trying is a 22/250AI; 1:8 twist 26 " barrel, with 75gr. Hornady match bullets. Those loads produced 3126 and then 3148 fps. Using "load from a disc" says optimally I should be at 3200 or better and with these lighter loads I have no pressure signs but am within a grain of their recommendations. I hope I'm just not fast enough but what do you think ?
Charles
 
I suspect that barrel doesn't stabilize that particular bullet, try a lighter bullet or a different brand in the same weight.

Drew
 
yep...lighter/shorter bullet...what is the twist ?


mike in co
 
charlie AR15 said:
Guys, this is a new one for me. I have a gun I'm working up loads for and set the target at 25 yds. for aiming purposes only so I could use my chrony. I noticed I was getting a comet tail on the bullet holes; but when I went up 1/2 a grain of powder, one shot went through sideways ! I know there's a simple answer as to why and how to solve the problem; but I've got a case of CRS and can't remember what it is ! Can anyone help me? I'm not much of a benchrester but knew you were and may be able to help. What I'm trying is a 22/250AI; 1:8 twist 26 " barrel, with 75gr. Hornady match bullets. Those loads produced 3126 and then 3148 fps. Using "load from a disc" says optimally I should be at 3200 or better and with these lighter loads I have no pressure signs but am within a grain of their recommendations. I hope I'm just not fast enough but what do you think ?
Charles
Click to expand...

Charles,
With your 8" twist, your bullets should be stabilized easily. When Hornady first came out with that bullet, they said it would take an 8" twist to stabilize it. SInce then, I have seen boxes of that bullet where they have printed 9" twist required. EIther way, your 8" should work fine. Especially with the velocities you are getting.

I have shot those bullets for years in my 22-250 AI with an 8" twist as well.

The comet tails you are seeing are generally made by bullets about to come apart (rpm's too high, rough throat, or bad jackets) but I have seen them also from bullets that were shot through paper at very close distances. And the keyholing you are seeing might also be because you have the target too close. Remember, the longer the bullet is, the longer it takes to quit yawing. Every bullet yaws immediately upon it's release from the muzzle whether it be a short 6mm 68 grain FB or a 75 grain .22 caliber VLD. The difference is the short bullet settles down quicker (thank goodness or 100 yard benchrest aggs would not be anywhere near as small as they are!) and the long bullet requires a few more yards to achieve it.

It's my guess that you are witnessing a pre-dampened yaw at 25 yards rather than an insufficiently stabilized tumbling bullet.

Incidentally, I get 3200 fps out of my current lot of powder with this bullet and this caliber.
 
Thanks guys for the quick responses ! Mike, it's a 1:8 twist. Drewh, I was thinking I'd have to go lighter; but I really, really wanted to shoot heavy ! Drat it. But, since I'm well below the velocity safely attainable; do you think more speed would stabilize it? I think I can run up only about 150 fps. If not, there's always the 55s. Thanks again.
Charles
 
A friend recently went through this with a 17 fireball varmint rifle. I believe you will find if you try to push the velocity more you will find that the bullet goes away in a puff of smoke somewhere between the muzzle and the target. A bullet of the same weight that is shorter may work, in his case going from a plastic tipped boat tail to a shorter flat base of the same weight cured the problem.

Good grouper does have a point also, that from the published data the bullet should work with that twist. have you checked the twist on your barrel? if there is a rough part of the bore that stresses the jacket then the bullet could be failing from that as well

Drew
 
Charlie,

That little spray of lead is coming out the side of the jacket as the bullet progresses toward in-flight failure. The bullets that show a comet tail at close range are not likely to reach even 100 yds before they pop into a cloud of lead dust. Driving them faster will increase the percentage that fail.

The factors that cause burst bullets are numerous and additive. Long barrels, long bullets (long bearing surface), barrel friction, slow powders, high pressures, rough throats, dirty barrels, high velocity, fast twist (high rpm), hot barrels, aggressive rifling profile, thin and/or brittle jackets are all insults to the bullet that contibute to in-flight bullet failure. You might change just one of the preceding factors and get back behind the problem but the easiest solution is to substitute a different bullet for this application. It doesn't mean there is anything wrong with the Hornadays. They just won't do what your asking of them in that rifle.

Greg
 
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I really would like to see some difinitive proof (high speed photography) that a properly stabilized bullet "yaws" for a distance once it leaves the muzzle. I am not a physics major by any stretch of the imagination...but if there is much muzzle exit yawing going on...I would think that the small groups that are being shot these days would hardly be possible.
 
Mark,

The use of card stock as a target at very close range is a technique for evaluating initial yaw. I don't know if there is a "standard" distance for "yaw cards" but 7 yards is commonly used. When you get oblong holes in the card that's a pretty good indication that the bullet is flying sideways and how sideways it is at that point. Best part is cards cost a lot less than high speed photo hardware. This is the simplest way I know to measure initial yaw.

Greg
 
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Berger Bullets did some testing on this subject that coincides with what Greg is saying.
The comet trails are actually molten lead spewing from a compromised jacket.
Hence the reason Berger now offers two jacket thicknesses in some of thier bullets.
The core is melting from excessive friction in the bore in relation to jacket thickness. Not necessarily the bullets fault, just one of those things. Some barrels can push bullet A at high velocitys and some can't.
Fouling is a large contributor to friction.

You definately want to do some good cleaning on that barrel. IMHO.
 
MarkR said:
I really would like to see some difinitive proof (high speed photography) that a properly stabilized bullet "yaws" for a distance once it leaves the muzzle. I am not a physics major by any stretch of the imagination...but if there is much muzzle exit yawing going on...I would think that the small groups that are being shot these days would hardly be possible.
Click to expand...

I'll bet if you run a search on youtube or google, you'll easily find what you're looking for. This is a very well documented and video taped external ballistic principle. Come to think of it, I saw a clip on the Discovery channel the other night of a slow motion .50 cal bullet leaving the muzzle and it looked like the dang thing was going to veer off into space!

ANother cool thing to watch is arrows being shot in slow motion. The way they bend and wobble and yaw it's amazing they can straighten out and fly well so quickly. If you ever get a chance, watch a bow pro paper tune his rig indoors at 20 or 30 feet. It is very intriguing.
 
goodgrouper said:
I'll bet if you run a search on youtube or google, you'll easily find what you're looking for. This is a very well documented and video taped external ballistic principle. Come to think of it, I saw a clip on the Discovery channel the other night of a slow motion .50 cal bullet leaving the muzzle and it looked like the dang thing was going to veer off into space!

ANother cool thing to watch is arrows being shot in slow motion. The way they bend and wobble and yaw it's amazing they can straighten out and fly well so quickly. If you ever get a chance, watch a bow pro paper tune his rig indoors at 20 or 30 feet. It is very intriguing.
Click to expand...

I have video of arrows comming off of my own bow. The physics of an arrow are vastly different from a bullet. The arrow physically bends at it's weakest point because of several factors which include multi-directional pressure applied to a unidirectional nock at the back of the arrow, the arrow being supported at variable distance from the pressure application combined with the flexible arrow shaft. Rotation speed of the arrow increases with distance while velocity decreases and is applied only by friction against the vanes, which are at the back of an arrow.
I venture to say that bullets do not bend in the middle and rotational stabilization is forced upon the bullet from the lands and grooves of the barrel.
My bet is that the most accurate guns discharge bullets with the least initial "wobble" As I said though I'm not a physics major. I have spent a few nights at the Holiday Inn Express though... :D
 
I have used these bullets in my 22-250 AI 28" barrel with less twist than yours. Sometimes the twist in barrels is less than expected. Check it. Mine was.

With the boat tail on the bullet muzzle blast gas may be destabilizing your bullet as it exits the barrel. Try a load with a faster powder. Try a flat base bullet as they are less destablized by the muzzle blast gases.

What is the range to your target? If the bullet is about to come apart using a moly coated bullet will help.

Bore scope your barrel. Hand lapping will often help. Smooth barrels will impart less heat energy into the bullet.

If you are loading the bullets with a jump into the lands try loading them with about .008 jam. You will have to adjust the load. Bullets seated for a jam sometimes go into the lands and barrel with less heat energy going into the bullet.
 
MarkR said:
I have video of arrows comming off of my own bow. The physics of an arrow are vastly different from a bullet.
My bet is that the most accurate guns discharge bullets with the least initial "wobble"
Click to expand...

Gyroscopic physics apply to any projectile that spins be it an arrow, a football, or a bullet. THere are certainly different properties and characteristics with bullets and arrows, but all spinning projectiles have some degree of nutation, precession, and other rotational physical properties whether it be primary or secondary. But, if properly stabilized, these phenomenon dampen out with time and end up flying faster, farther, and more accurately than they would without them.

I know of a Texas bullet maker who experimented with jacket lengths and bullet shapes in 68 grain configurations with the intent of finding out which bullet design settled down the quickest. Because of the shortness of all the 68 grain bullets (comparitively), they all dampened before 100 yards but some were "going to sleep" at 50 yards and others were much closer than that. IN any case, a barrel (or more importantly) the crown of a barrel is a big contributor to the release of the bullet and it's initial "yaw". But even a perfectly cut crown cannot prevent initial "yaw" of a spinning projectile. Once it leaves the confines of the barrel, external ballistics and physics take over.
 
Octopus said:
With the boat tail on the bullet muzzle blast gas may be destabilizing your bullet as it exits the barrel. Try a load with a faster powder. Try a flat base bullet as they are less destablized by the muzzle blast gases.

What is the range to your target? If the bullet is about to come apart using a moly coated bullet will help.

Bore scope your barrel. Hand lapping will often help. Smooth barrels will impart less heat energy into the bullet.
Click to expand...


??? Can you provide your source of info on the first paragraph? I have read tests about this and never did any of them come to this conclusion until the angle of the boat tail and the caliber of the boat tail in relation to the caliber of the bullet was disproportionally extreme. Regular boat tails of current design do not have such radical shapes.

The second sentence could be true if your getting jacket disruption from the bore as a result of excess friction. However, if the jacket was coming apart because of excessive rpm's or jacket flaws, moly isn't going to help at all. Come to think of it, the last bullet that came apart on me was a moly bullet!


Your third sentence is also not recommended by most barrel makers. Hand lapping is a delicate matter and shouldn't be done at all without excellent equipment and lots of experience doing it. If you "garage" it, you could actually RUIN a barrel.
 
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