Keyhole

Aloysius · Posted: Tue Sep 21, 2010 12:09 am Post subject: Re: Keyhole

I think you're pushing that bullet too fast, so it strips trough the lands and ruin the shape of the bullets which makes it tumbling.
When your twist rate is too fast for a small bullet, decrease speed. That's easier done than when your twist rate is too slow for a big bullet, pressure limits don't allow to increase the speed till the required level.

When you could recover such a keyholing bullet, I think you would see a ruined jacket. Also check your barrel for built up jacketmaterial.

slimjim · Posted: Tue Sep 21, 2010 4:29 am Post subject: Re: Keyhole

Pkslinger, I found you calculationsn on key hole interesting. I think the angle should be calculated from the last 10 yards of trajectory to be more representative. Wouldn't that be a drop of about 10 inches over 360 inches. Also, once the bullet starts to yaw, oops wrong term, slip into the wind, wouldn't that slip angle be increased by aero forces such that the angle would be increased over the angle from rifle barrel exit to trajectory at target impact?

Where is Rinker's book when you need it?

Pumpkinslinger · Posted: Tue Sep 21, 2010 8:27 am Post subject: Re: Keyhole

Thanks to PointBlank I could look at the angle for the last 5 yards and got 1.62 degrees. I first used the angle for the last 50 yards and got 1.56 degrees. All in all not much difference.

Here is some info I found online that influenced my thinking on this subject: www.navweaps.com/index...ch-043.htm

I guess we might need to define terms better. When I say over-stabilized I mean that the gyroscopic effect on the bullet won't let it nose over like we want. The bullet stays pointed in the direction perpendicular to the axis of the spin. It appears to me that when people say "over-stabilized" they are really talking about pushing a bullet into a faster rotation than its construction will allow, causing it to come apart in the air. That's not really the same thing.

slimjim · Posted: Tue Sep 21, 2010 7:09 pm Post subject: Re: Keyhole

Pumpkinslinger wrote:

I guess we might need to define terms better. When I say over-stabilized I mean that the gyroscopic effect on the bullet won't let it nose over like we want. The bullet stays pointed in the direction perpendicular to the axis of the spin. It appears to me that when people say "over-stabilized" they are really talking about pushing a bullet into a faster rotation than its construction will allow, causing it to come apart in the air. That's not really the same thing.

I share the same definitions with you. These two occurances are not the same thing but are both consequences of overstabilization. In the later, the centrifugal forces destroy the bullet before it gets the chance to exhibit the former.

Thanks for the article, it was interesting. If Dawgdad still finds some of the TNT bullets "keyhole" at 100 yards, I think it might be informative to shot some rounds at 200 yards to see if there is any changes.

I would think it would be interesting if

SingleShotLover · Posted: Wed Sep 22, 2010 4:52 am Post subject: Re: Keyhole

For clarity sake, I mentioned over-stabilization as a contributing factor, not the whole problem. When paired with a defect such as mentioned (dirty bore, burr and etc.) key-holing is a strong possibility. Sounds to me that the bullets are on the thin line between instability and destruction...but then without actually experiencing the problem, I'm just guessing

PaulS · Posted: Wed Sep 22, 2010 3:37 pm Post subject: Re: Keyhole

I don't believe in "over-stabilizing" a bullet.
Now that, to me, means spinning a bullet to fast for its length.
As long as the bullet stays together it can't be "overstabilized at the ranges that I shoot. If the nose doesn't lean over into the airflow it is a minimal concern.
I have fired a 158 grain 35 caliber pistol bullet at 2700 fps from a 1 in 12 twist at 100 and 200 yards resulting in .3 MOA groups. No key-holing and no ill effects on the bullet, its flight, or the impact on the paper target.
Would I use it to hunt deer with? not unless I was going to shoot for head or neck shots - I don't believe the bullet would hold up long enough at that velocity for any kind of body shot. I haven't had the opertunity to test it but I don't like the physics of it at all.

slimjim · Posted: Thu Sep 23, 2010 2:36 am Post subject: Re: Keyhole

Pumpkinslinger wrote:

Well, if it destabilized its not over-stabilized...

Concur. Will be interesting to see how Dawgdad tames this rifle for this very light bullet.

Dawgdad · Posted: Thu Sep 23, 2010 7:19 am Post subject: Re: Keyhole

I tink the answer is to clean the bore to make sure I have good rifling gripping the bullet and slow it down just a touch so that it has a chance to fully engage instead of skidding down the bore... While all of the calculations are fun to ponder - I am going to use Achem's Razor here and say the keyholing was due more to a fouled bore and a bullet with a small bearing surface not becoming fully engaged in the rifling and "sliding" through some parts of the bore. Without recovering some of the keyholed bullets I will not know for sure but cleaning the bore and slowing down the speed has shown no keyholes yet.

Pumpkinslinger · Posted: Thu Sep 23, 2010 7:45 am Post subject: Re: Keyhole

Dawgdad, I think you're dead on with your solution. The number crunching was just a way to show why I didn't think "over-stabilization" was the problem.

slimjim · Posted: Sun Sep 26, 2010 7:20 pm Post subject: Re: Keyhole

Dawgdad, post some of your targets if you can.