Keyhole

Dawgdad · Posted: Sat Sep 18, 2010 5:59 am Post subject: Keyhole

I have a 30-06 load with 125 Speer TNT bullets that I have used successfully in my model 70 for years. My nephew has a Savage 110 and needed a coyote load so I gave him a box of these. When we went to check the zero about 25% of the shots were keyholing at 100 yards.

The load is near max so would you suspect that his chamber is a bit tighter than mine causing higher pressure/velocity thay upsets the TNT's balance?

Pumpkinslinger · Posted: Sat Sep 18, 2010 7:18 am Post subject: Re: Keyhole

Wow, that is odd! I looked at the Speer manual to see what the max velocity for that bullet was and its 3100 ft/sec. I guess its possible you're exceeding that and getting some deformation when the bullets exits the muzzle. However, the first thing I'd check would be the cleanliness of the bore. It looks like both rifles have the same twist, 1 in 10". www.exteriorballistics..._Rifle.pdf

Bushmaster · Posted: Sat Sep 18, 2010 8:05 am Post subject: Re: Keyhole

You might also check the crown on that rifle. A slight ding at the muzzle can cause that too.

I don't have a tumbling problem with the lighter bullets in my Browning, but I seem to lose accuracy with 130 grain and lighter.

SingleShotLover · Posted: Sat Sep 18, 2010 8:17 am Post subject: Re: Keyhole

Is the angle of the keyhole random or uniform? That's a pretty short bullet for 1-10" twist and, contrary to popular belief, it is possible to over stabilize a bullet. This causes a bullet to fly with a slightly nose-up angle which could be exaggerated by either a fouled bore as Pumpkinslinger suggested or as Bushy mentioned, a small ding in the crown.

slimjim · Posted: Sat Sep 18, 2010 3:05 pm Post subject: Re: Keyhole

Wow, on two accouts!! I checked the Savage website to verify twist - I didn't know they had so many models. Second, with a 1:10 twist and a bullet length of 0.920 inches, the stability factotr is over 4.2!! Even with the longer bullets, the stability factor was over 2.0 most of the time. I double checked my calculations and even used a different calculator. I didn't know that .30-06 was such an overstabilized caliber.

So, we have an overstabilized bullet that is getting destabilized. I've heard some suggest that if the shockwave gets to the end of the barrel at the same time the bullet exits, it could be upsetting. Is this a possibility?

chambered221 · Posted: Sat Sep 18, 2010 5:59 pm Post subject: Re: Keyhole

Some of the earlier 308's and 30-06's did have a 1-12 twist !!! What does your model 70 have ???

Over-stabilization was the first thing I thought of too, although there are some that would argue it's not possible.

Studies have proven that the normal twist rate (1-10) for these calibers are not ideal for the lighter bullets, especially those under 1 inch. The 1-12 and 1-14 twist is better suited for the light short bullets but has given way to the more popular 150-200gr that typically are in the 1.1 - 1.3 inch range.

If your twist rates do differ you could try loading a few at a start/minimum load and see what happens.
By doing this your slowing down the rpm of bullet. Not the twist rate, but the speed of the revolutions. This trick may only work if your on the edge of stability to begin with.
Dropping the velocity with this bullet may not be such a bad idea in itself. To my understanding their thin jackets don't tolerate max pressures and velocities all that well to begin with.

Let us know what you find !!!

Dawgdad · Posted: Sat Sep 18, 2010 7:54 pm Post subject: Re: Keyhole

Thanks for all the input. I usually associate keyholing with too little velocity not stabilizing a heavier bullet.

PLAN OF ATTACK!

We fired about 5 shots with military ball at 25 yards to get the scope printing on paper and they were fine at that distance. We will try them at 100 yards too.

I will :

check the twist on both guns.

Q-tip the crown looking for a burr. I spun it in the bright sun and did not detect any shiny spots with my 50 year old eyes.

JB paste the bore.

shoot the load in my gun again.

shoot some 110 V-max in his gun.

If those are all positive we will try ther TNT's again

Stay tuned for further developments.

44marty · Posted: Sun Sep 19, 2010 12:15 pm Post subject: Re: Keyhole

Playing with .224 bullets in sabot loads in a Rem 700 30-06, I found this problem raised its ugly head also. Too much spin (high RPM) started early, with lighter loads keyholing - around 3800 fps. When pushed to the max at well over 4000 fps, bullets actually disintegrated with a gray puff seen 30 to 50 yards from the barrel.
The same thing happens with under-stabilization (55gr long bullets in a .204 ruger require a 1 in 8" twist).

Elvis · Posted: Sun Sep 19, 2010 9:21 pm Post subject: Re: Keyhole

imho over stabalization is more a case of the projectile being pulled apart by centrifugal force. do the maths and you will see what I mean. for example 1 in 12 twist doing 3000fps (just cause the maths is easy)
3000 rotations per secound x 60 secounds =180 000rpm
now thats got to be a plurry large amount of outwards pressure on a projectile and any slight imperfection would cause problems.

Pumpkinslinger · Posted: Sun Sep 19, 2010 9:47 pm Post subject: Re: Keyhole

Over-stabilization is a problem with artillery rounds, not sure it is with rifle bullets. If the projectile is over-stabilized it won't nose over as it goes through its downward trajectory toward it's target. I think there is something about that in Rinker's book but I'll have to find it.

slimjim · Posted: Mon Sep 20, 2010 4:18 am Post subject: Re: Keyhole

Pumpkinslinger wrote:

Over-stabilization is a problem with artillery rounds, not sure it is with rifle bullets. If the projectile is over-stabilized it won't nose over as it goes through its downward trajectory toward it's target.

This is a problem with rifle bullets also. I've read it starts to show itself after 500 yards. GS Custom Bullets stresses this if you intend to shoot game beyond 500 yards so the hunter doesn't introduce bullet yaw as a destabilizing factor in the terminal performance of the bullet.

Dawgdad · Posted: Mon Sep 20, 2010 7:01 am Post subject: Re: Keyhole

Follow up:
Q-tip the crown looking for a burr. I spun it in the bright sun and did not detect any shiny spots with my 50 year old eyes. -Result - crown showed some wear but no obvious dings. I pushed a bullet into the muzzle and all land marks were even.

JB paste the bore. - I wish I took a picture of the patches! - Butches and Kroil first - let it soak... lots of copper came out - clean patches two swipes later... ran JB patch... 4 nasty black gunk filled patches. hit with butches and Kroil - two patches later clean..

Shoot the load in my gun again. - I did not have enough bullets to do this step.

shoot some 110 V-max in his gun. - I shot this load in my gun and while a max load it showed no high pressure sign and out two bullets in the same hole at 200 yards. - when fired in his gun - bolt ws hard to lift and group was wild at 100 yards.

I fired the greek military ball ammo and it was a very tight group at 100 yards.

If those are all positive we will try ther TNT's again - I only had four loaded rounds with me but all four were round holes with no keyholing.

Conclusions: His chamber is tighter than mine and this load and the V-max load are too hot for his gun. I will need to back them down and work up for his chamber. The dirty bore added to the bullet instability.

Pumpkinslinger · Posted: Mon Sep 20, 2010 11:37 am Post subject: Re: Keyhole

My trigonometry is pretty rusty but if I'm figuring right (and with some help from PointBlank):

EXAMPLE: .30-06, 168 grain boat tail Match bullet at 2800 ft/sec with a 1,000 yard zero.

The angle the bullet leaves the barrel is 0.611 degrees above horizontal. (arctan of 384" of total drop / 36000 inches of distance) So if the bullet was over-stabilized so that bullet nose didn't drop at all it would hit about 2.2 degrees off (angle of bullet's descent plus angle due to over-stabilization). In other words, I don't think you'd be able to detect a keyhole due to over-stabilization, even at 1,000 yards.

Dawgdad's conclusion looks good, I'll be looking forward to the next tests!

slimjim · Posted: Mon Sep 20, 2010 8:40 pm Post subject: Re: Keyhole

Would'nt the issue with overstabilization be if the bullet has any imbalance, the higher than nominal spin rate could cause it to destabilize and not hit the target nose on?

Pumpkinslinger · Posted: Mon Sep 20, 2010 11:12 pm Post subject: Re: Keyhole

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