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Posted: 11/30/2007 6:00:29 PM EDT
Went to the range yesterday and shot about 400 rounds. I noticed that with shooting one particular brand of .223 62gr FMJ ammo at 100 yards I was getting hits that showed a lot of bullet yaw. Some even looked like they were going in sideways. I switched brands and went to a 55gr pill and it looked better. I have a 1:9 twist unlined barrel with about 2000 rounds through it. Any thoughts?
Link Posted: 11/30/2007 6:04:41 PM EDT
keyholing is caused by your gun not stabilizing the bullet you're shooting.


"one particular brand" of ammo does nothing to help us answer your questions.


let me consult my magic 8 ball to see if it can help.
Link Posted: 12/1/2007 6:43:42 AM EDT
height=8
Originally Posted By Zack3g:
keyholing is caused by your gun not stabilizing the bullet you're shooting.


"one particular brand" of ammo does nothing to help us answer your questions.


let me consult my magic 8 ball to see if it can help.


Thanks for no help Zack. It's a physics questions not a who made the ammo question. I asked what causes it. Why is the bullet not stabilizing? Is it under powered? Is it too heavy? Is the twist not enough? I left out what kind it was so we don't get more brand bashing. You didn't ask what brand of gun I have or what kind of coffee I drink. You could consult your magic 8 ball but it may be hard to see with you head in a dark and smelly place.
Link Posted: 12/1/2007 7:09:29 AM EDT
[Last Edit: 12/1/2007 7:11:00 AM EDT by mdwicks]
Now that we have the requisite dumb ass post, in this thread it is from the ever-mature Zack, maybe we can get a real answer.

The only thing I can think of is something in the path of the bullet, like a leaf or some blades of grass or something. I know that things that small could cause yaw, but other than that I have no idea.

It might help to know what weight the other brand of ammo you were shooting, though.
Link Posted: 12/1/2007 7:26:10 AM EDT
[Last Edit: 12/1/2007 7:31:06 AM EDT by ChromeLined]
Maybe the throat is starting to erode.Keyholing is caused by an unstabilized bullet...worn or damaged rifling or worn throat 2k rounds is a small round count for that to happen.Heat and how abused the barrel gets can be a factor as well..any damage to the muzzles crown?and I agree if no one can post help to a problem then just dont post...

I wish I had a crystal ball to see if your under 12 years old because it sure sounds like it.to zak3g "duh" obviously the bullets not being stabilized maybe if you had more knowledge and posted why that may occur wed be closer to helping this person...you went off track on the ammo...he gave an effect and there are causes that make tha happen so your smart ass comment simply replaced smart...you dont need a crystal ball when an effect is given...iff you dont have any possible causes just dont post.
Link Posted: 12/1/2007 7:44:07 AM EDT
Link Posted: 12/1/2007 8:54:20 AM EDT
Are you sure that you have a 1/9 twist , I mean is it roll marked on the barrel?

A 1/9 should have NO problem stabilizing a 62 or a 69 gr bullet. A 1/12 will not.

If your barrel is not roll marked you can put a bore brush on a rod and start it in the bore. Mark the starting point and put a index mark on the rod , puss in for 1 revolution and mark the rod. Then pull it out and measure the distance between the two marks and that is your twist rate.

Link Posted: 12/1/2007 8:55:41 AM EDT

Originally Posted By foxherb53:
Are you sure that you have a 1/9 twist , I mean is it roll marked on the barrel?

A 1/9 should have NO problem stabilizing a 62 or a 69 gr bullet. A 1/12 will not.

If your barrel is not roll marked you can put a bore brush on a rod and start it in the bore. Mark the starting point and put a index mark on the rod , puss in for 1 revolution and mark the rod. Then pull it out and measure the distance between the two marks and that is your twist rate.



+1
Link Posted: 12/1/2007 8:57:41 AM EDT
height=8
Originally Posted By foxherb53:
Are you sure that you have a 1/9 twist , I mean is it roll marked on the barrel?

A 1/9 should have NO problem stabilizing a 62 or a 69 gr bullet. A 1/12 will not.

If your barrel is not roll marked you can put a bore brush on a rod and start it in the bore. Mark the starting point and put a index mark on the rod , puss in for 1 revolution and mark the rod. Then pull it out and measure the distance between the two marks and that is your twist rate.



Thanks for the help...The barrel is 1/9 and it's only about 3 months old. It's been cleaned frequently. I believe the problem is the ammo. It may be a combination of the QC of the ammo producer and the headspacing of the 5.56 chamber shooting that particular .223 round. I moved to UMC 55 gr ammo and it was much better. Next I will go try some Federal 5.56 55gr FMJ that I have and report back.

Thank you for your support.
Link Posted: 12/1/2007 9:03:08 AM EDT
I am working from my memories of physics here. I probably know only enough to be dangerous, but here goes:

Stabilization is a tug-of-war between those external forces acting on your bullet and the inertia created by rotation. If the inertia is greater than or equal to those external forces, the bullet is stable, if not, it will precess, yaw or tumble.

The inertia (resistance to acceleration) is created by a phenomenon known as angular momentum, which is created by the spinning of the bullet in the rifling.

The external forces are largely the result of resistance of flying through the medium. A bullet does not "want" to fly point forward, because the higher surace area/ mass ratio of the nose creates more drag that encourages the nose to trail the base ( the bullet wants to fly backwards). The denser the medium, the greater the forces of drag, and the greater tendency to flip the bullets around. This is why bullets can be stable in air, but yaw in flesh.

Another big event in a bullet's life is exiting the the bore. In the barrel, the bullet is forced to spin about its geometric axis, which is the physical centre between the bearing surfaces. In air, though, it must spin about its centre of mass. If a bullet is not concentric, it will shift its centre of rotation in the first few metres after leaving the bore. A large shift at this point may induce wobbling from which the bullet will never recover, leading it to precess or yaw in flight.

That then gives us three things that may cause destabilization in flight:
- medium being travelled is too dense
- medium is not of consistent density. A network of twigs in a matrix of air would be an example of an inconsistent medium
- A poorly formed bullet that has a geometric centre that differs considerably from the mass centre.

Link Posted: 12/1/2007 9:19:38 AM EDT
you may also want to remove your flash hider and inspect the barrel crown.

if you have a build up of carbon, either on the crown or on the flash hider you 'may' have a clue as to why.

but this is kind of a long shot since it's not happening with every brand.

I would suspect that you were shooting a poor quality bullet from a "discount" manufacturer.
Link Posted: 12/1/2007 10:45:43 AM EDT
Masters, One and all. Stormwalker, I think you hit the bullet on the head. I was using imported cheap ammo. I figured it was probably the case but couldn't explain what it was that would cause it to fly so poorly. The inconsistant weight is my guess. Anyway, I'll know what to expect when shooting crap ammo next time. Thanks.
Link Posted: 12/1/2007 11:15:40 AM EDT
[Last Edit: 12/1/2007 11:16:28 AM EDT by Taskism]
I was reading something in Guns & Ammo the other day that some dude wrote about the end of your barrel getting tweaked up a bit from it touch the ground or whatever and it can screw up the seal where the bullet comes out of the barrel and the pressure isnt the same on all sides and it causing bullets to keyhole a bit.

They had some technique in there to fix it but I don't know where magazine is or Id give you more information.

They coated the end of the barrel with white out and fired a round and looked at the burn marks to see where more pressure was escaping from though that messed up the spin. How they fixed it I don't remember.
Link Posted: 12/1/2007 11:53:52 AM EDT

Originally Posted By Taskism:
I was reading something in Guns & Ammo the other day that some dude wrote about the end of your barrel getting tweaked up a bit from it touch the ground or whatever and it can screw up the seal where the bullet comes out of the barrel and the pressure isnt the same on all sides and it causing bullets to keyhole a bit.

They had some technique in there to fix it but I don't know where magazine is or Id give you more information.

They coated the end of the barrel with white out and fired a round and looked at the burn marks to see where more pressure was escaping from though that messed up the spin. How they fixed it I don't remember.


Fixing a crown usually involves cutting the end of the barrel back on a lathe or with other specifically designed crown cutting tools.

As to OP's question. Heavier bullets are usually longer. LONGER bullets need to be spun faster to stabilize. I think your problem would be caused by a lack of uniformity in jacket thickness or some other "bullet" deficiency.
Link Posted: 12/1/2007 12:24:36 PM EDT
I seem to recall ammo oracle talking about the heavy end of the bullet tending to want to come around. So the back ends up going first if given enough time and not enough stablization. Tried to find it but the site (or my pc) is acting strange.
Link Posted: 12/1/2007 12:27:43 PM EDT

Originally Posted By balldragger:

Originally Posted By Zack3g:
keyholing is caused by your gun not stabilizing the bullet you're shooting.


"one particular brand" of ammo does nothing to help us answer your questions.


let me consult my magic 8 ball to see if it can help.


Thanks for no help Zack. It's a physics questions not a who made the ammo question. I asked what causes it. Why is the bullet not stabilizing? Is it under powered? Is it too heavy? Is the twist not enough? I left out what kind it was so we don't get more brand bashing. You didn't ask what brand of gun I have or what kind of coffee I drink. You could consult your magic 8 ball but it may be hard to see with you head in a dark and smelly place.


the physics of why keyholing happens is not as important as why it is happening with your gun. knowing what ammo you're shooting and what twist rate your barrel is will help those of us who are familiar with the weapon system to determine your root cause.

but you're absolutely right. knowing what type of ammo you're using wouldn't help us to know if the ammo is underpowered or too heavy or if your twist rate is wrong for the ammo you're shooting.

Link Posted: 12/1/2007 1:17:08 PM EDT
Good stuff. Concentricity is the answer to most practical stability questions. The difficulty is determining where you're not concentric. Poor bullet construction results in a bad spiral. Poor ammo assembly or throat erosion results in the bullet not engaging the rifling in line with the bore. A damaged or dirty crown or a dinged flash hider (not likely) can cause a change in the pressure around the base of the bullet as it leaves the barrel, all resulting in wobble.

it sounds like a bad ammo issue. Most good 1/9 barrels will stabalize even 77 grain bullets reliably, if they're loaded properly. No barrel will stabalize a junk bullet in any weight. BTW, tumbling bullets can do a ton of damage. They often expend their energy on target without pass through. They just don't hit as often from beyond close range.
Link Posted: 12/1/2007 1:53:35 PM EDT
Try a google on Greenhill Formula.

150 X Bullet Diameter Squared
------------------------------------ = recommended twist rate ( or close to it)
Bullet Length


I have had a gun keyhole with very worn throat, but 2000 rounds does not seem excessive.
Another thing to check is crown of the barrel, damage to the crown can create keyhole problems.
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