Key-holing

I have known for a long time that often something strange happens out between 30 and 40 feet when shooting my STI Eagle 5.1 (chambered in .40 S&W). The groups get much larger. At about 20 feet I can almost keep it on ragged hole even when shooting unsupported. But at 40 feet it’s about 6” to 8” groups. At 75 feet they will just barely stay on an USPA target.

I went to the range today and finally figured it out. Below are three different bullet holes from a target at 75’ feet.

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Some of the bullets are key-holing the target. This means the bullets are not stable in flight.

The same thing could be seen at 40’ but less frequently. On the 40’ target, which used a different type of paper you could actually see an imprint of the side of the bullet.

This was cheap “gun show ammo” in 180 grain FMJ but I’m pretty sure it happens with at least some of my hand loads too.

I then tried some Winchester Ranger in 135 grain HP. It was a 16 shot 2” group at 40’. I tried Winchester Ranger in 180 grain HP with the same result; a 2” group at 40’.

I have some 180 grain Montana Gold HPs that I should load up and see if they give me the better results. Fellow shooter Don W. told me a year or more ago that he did experiments with various bullets with his STI chambered in .40 S&W and the Montana Gold 180 grain HP gave him much better accuracy than the FMJs.

I think I will have a chance to do the tests next weekend.

5 thoughts on “Key-holing

  1. Hmm. It occurs to me that a hollow point should have a higher rotational inertia than a FMJ with the same mass. It would thus take more torque (from air friction) to slow down its rotation to the point it became unstable, and thus, given the same muzzle velocity and rotational speed, should travel further before starting to wobble.

    Is that plausible?

    -m@

  2. What’s the twist rate in STI barrels vs everyone other’s barrels? And, is the STI barrel conventional, Ballard or polygonal rifling? Tried any other barrels?

  3. @matt d, If the bullet were hollow all the way through you would be correct. But it is just the front part of the bullet that is hollow. There is another consideration here. The front part of a hollow point bullet is lighter than the rear. Think of an arrow. The head is heavy and the tail is light with a lot of drag if it goes sideways. A bullet always rotates about its center of mass. With a hollow point the center of mass is move rearward compared to the overall length of the bullet. If the bullet has more drag ahead of the COM than behind the COM then, absent spin, it will turn and travel base first. Hollow point, in general, are going to me more prone to do this. But the overall profile of the bullet matters. For a given spin rate it’s all about drag relative to the COM.

    @Larry, The barrel is a KKM Precision barrel. I do not know the twist rate. It is a conventional rifling. I have not tried other barrels in recent years. I had an STI barrel in it originally but it broke (probably incorrect installation). In this style of gun barrel replacement is a gunsmith job. You can’t just drop in a new one. It has to be fitted to the gun.

  4. Maybe it just likes 135s better than 180s. There’s a pretty large velocity difference. The heavier bullets go slower and so they turn slower. Might need to stick to 155s or lighter in that gun. Or get a 10 mm or 45 if you want to sling heavier lead.

  5. I saw this exact problem with factory ammo in a .300 WinMag this past weekend. New rifle, with 50 rounds down it. BUT he was keyholing at 300 and 600. NO WAY his velocity wasn’t sufficient to keep the bullets rotating correctly at those ranges.

    I lean towards a barrel problem, but….

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