The Best Keyhole Example Ever!

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This morning I was out shooting at our local range 100 and 200 AR challenge. AR-10's shoot 200 yards, AR-15'S shoot 100 yards.

AR-10 class includes .308, 6.8 spec, .243, 7.62x39 etc

AR -15 class includes .223, .300 blackout and the like.

Well, one guy showed up with a new 300 blackout and some factory 150 gr super sonic rounds. His barrel was a 1:8 twist.

He couldn't hit the target at 100 yards. He said it was sighted in at 100, but he couldn't hit the paper anywhere. He kept shooting, trying to hold in different places but no luck.

Then, we went and retrieved targets, which look like this:

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If you look closer at the target on the low right, you may be able to diagnose his issue:

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This is the "best" keyhole I have ever seen.... you can literally tell the caliber, length, and shape of the bullet fired.

I don't know the guy so I didn't spend much time helping figure out why this was happening. But I have never seen a keyhole that perfect.
But we do know why he couldn't hit the target.
 
Saw that with a drill rifle that had the chamber eaten out by firing blanks.

No such thin on it as throat erosion, gauge went all the way.

My brother wondered if you could get it to shoot at all (25 yards key holes all over the map

I gave it some thought and I had some 200 grain flat base 308. Loaded those up as slows you could get

We got it to do a 4 inch group at 50 yards.

Key hole is an unstable bullet.

Too slow, eaten out throat, wrong bullets (too small)
 
I would love to know what was causing that.

He should have no problem at all shooting 150 grain bullets out of a 1:8 twist barrel.
 
How heavy were his bullets. I'm not a pro here, but a 1:8 twist may not be enough for the heavier bullets. I had a Rem 7 bolt action in 223. Shot 55 gn bullets just fine. Loaded up some 68 gn and they key holed. The Rem7 had a 1:8 twist. Moved up to a Savage with a 1:12 and the heavier bullets shot fine. It was a Savage 11. Thing was a tack driver. Recently sold it off for lack of use.

Life is good.
Prof Young
 
He should have no problem at all shooting 150 grain bullets out of a 1:8 twist barrel.
Click to expand...

Oh I agree. Like I said I don't really know the guy. But if he asked me, I would have told him to try some different factory ammo first, in case the bullets in this case were somehow under sized for a standard bore.

But if that doesn't work, and 150's won't stabilize in a 1:8 twist, then it's time to check chamber, rifling, bore diameter, etc

@ Prof Young, they are 150's
 
Wow. I do wonder what the cause is for that. My 1/8 300 blk has been shooting 150's over 12 gr of 2400 pretty well. I have read accuracy horror stories with 300 blk but mine has driven tacks with supers from 120 to 150 gr.
 
I know you said you don't know any details on this but, by chance do you know if these were jacketed bullets ? Or cast bullets ?
 
That is weird.
Really weird.

Keep your ear to the ground for any updates. I would LOVE to hear what exactly happened. I can't come up with any reasonable explanation.
 
Query:
What weight were the previous bullets that shot just fine? Like I said before I am not a pro here, but my experience with key holing bullets had to do with weight and twist rate. I could be way off here . . . so . . .

Life is good.
Prof Young
 
What weight were the previous bullets that shot just fine? Like I said before I am not a pro here, but my experience with key holing bullets had to do with weight and twist rate. I could be way off here . . . so . . .
Click to expand...

You are correct, assuming that the projectile is the proper diameter for the bore, stability is based upon gyroscopic principles. Essentially a 3-D gyro spins and resists an apposing force attempting to alter the spin plane (the opposition is at right angles to the applied force).

In fact, when I was on DDG-57 while in the Navy, navigation gyros might be facing down at noon, then at midnight be facing straight up (assuming the ship didn't change cords much). What happened was, the gyro didn't move, the earth turned and so the gyro was facing the exact same point in space that it was before.

Back to bullets, when bullets get heavier, they get longer since they can't get fatter. The longer they are, the more the back end of the bullet wants to pass the front end. Gyroscopic stability prevents this from happening and the faster it spins, the more resistance the bullet offers.

But if it isn't spinning fast enough, other forces win out and it tumbles.

Also, bullets can tumble when they go through what is called the transonic zone where the nose is breaking the sound barrier, and the rear is not. The bullet must be very stable to make it through this zone. (Better sectional density helps ).

And you can't really over spin the bullet, provided it doesn't fly apart due to centripetal force.

But, in this case, a 1:8 twist at super sonic speeds should easily stabilize a 150 gr .30 cal bullet.

Perhaps it is a bad barrel/chamber or perhaps it is bad ammo. If I see the guy again, I may ask him what came of it. But at the competition I didn't have time and he was leaving .....and kind of pissed I think, so I didn't press
 
Something to do with the gun.

Bullets may go a tad unstable, but not that unstable.

Long throat maybe.
 
I have ran similar Federal 150FMJs through my 300blk, they seemed quite underpowered to me. I never checked them side by side but my reloads shot quite a few inches higher at 100yds.
 
I shoot 150g Speer SP from my 300 Blk all the time and it's a tack driver.
I have a 16" Ballistic Advantage barrel with a 1:7 twist.

With the 1:8 twist, the only way it should be key-holing is maybe it's an SBR or a pistol and has a short barrel? I shoot 220g Berries subs in mine too and they don't keyhole and are very accurate as well.

Excessive velocity can cause it as well but that seems unlikely in a 300 BO with factory ammo.

Interesting.
 
So let me ask this . . . back when I had the keyhole problem with my Rem 7 in 223 shooting heavier bullets, if I had made a faster yet still safe load, might that have solved the problem?
Life is good.
Prof Young
 
Prof Young So let me ask this . . . back when I had the keyhole problem with my Rem 7 in 223 shooting heavier bullets, if I had made a faster yet still safe load, might that have solved the problem?
Click to expand...


Theoretically yes. But practically, Probably not. It takes a lot of extra velocity to make up for less twist. We see this with the following equation:

Bullet RPM = MV X 720/Twist Rate (in inches)

So let's say hypothetically that an 80 gr match king is only just stable in a 1:8 twist AR doing 2650 fps which is a fairly stout load in a carbine.

The bullet RPM in this case is thus 238,500.
In a 1:9 twist the rpm is 212,000

If we hold the RPM constant, and do a little algebra, we find it would take 2981 fps in a 1:9 twist to achieve the same RPMs of a 1:8 twist at 2650 fps.

If it's a bullet better suited for a 1:7 twist, the necessary velocity to get the same rpm from a 1:9 twist is therefore even higher.

So yes, in theory...but in practice probably not.

I would say you can improve on marginal stability, but if it is keyholing, no you can not stabilize that with velocity
 
3 BMG Keyholes

Here's 2.5 BMG rounds that keyhole'd. The top one on the right side of the middle pallet slat is pretty obvious that a 660 grain round hit sideways.

Msob0Ib.jpg
 
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Heck yes, that picture has to easily be the coolest keyhole picture I have ever seen. And while accuracy would be horrendous... I wouldn't even want to see the wound on a short range live flesh/muscle target. That would do gruesome damage.
 
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