What makes a bullet accurate?

I’m not an engineer but would think complete bore obturation would be absolutely essential, which would come back to bullet hardness, which I guess would lead back to alloy selection. Some of the early Barnes mono bullets had a reputation for excessive fouling, I suppose because the alloy was soft to allow obturation. Barnes eventually went to a coating and then the TSX with their driving bands to combat fouling. So apparently the balance between hardness and obturation is quite critical. Also, mono bullets are long for their weight, so twist rate can be a concern. Any chance your barrel isn’t completely stabilizing the bullets? Accuracy potential is also affected by barrel harmonics, which is what we are affecting by “tuning” our load with powder charge and seating depth. No matter how finicky a bullet is, I would have a hard time believing a .1 gr difference in charge weight would make the difference you are seeing on target. How many shots per charge are you testing? Single three or even five shot groups are not a large enough sample size to be statistically relevant. Multiple groups of three or five or single groups of a larger sample size are needed to determine a true difference in potential.

John

Where did your bullet design come from? Bryan Litz (and many others) wrote about bullet design including elements like center of gravity. Since you aren't using jacketed lead bullets you probably can't use a lead bullet shape.

If I recall correctly the shape of the nose and boat tail effects drag so those elements matter more at long range and less at short range. I can tell you that meplat diameter doesn't matter much at 100 yards. I used to make 6 mm flat-based bullets for my BR guns. Mine had large meplats (IIRC about .060, about the same as the ejector pin) but they shot fine at 100 and 200 yards.

The reason for the boattail is to smooth the air after the bullet passes and creates a stable pocket of air behind the traveling bullet.
Not having that will cause turbulence behind and will un-stabilize the bullets travel forward.

It would be better to have a high strength tool steel mold to compress the bullet into after machining it.
If nothing else it would make the bullet it's self have a more stable and consistent grain structure making it better balanced.
Forging vs. billet I guess is the best summation.

The shooter

secondofangle2 said:

The shooter

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This.

secondofangle2 said:

The shooter

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Nah.
A world class shooter in a world class rifle cannot compensate for an inaccurate bullet.
Just try and get a sub MOA group from 62 grain green tips some time.
You *might* get lucky 1 group out of 1000.
I've seen some pretty shit bullets over the years.....still have a few boxes of late 50's, early 60's Nosler Partition 30 cal 150 grainers that are machined, visible tool marks.
I don't care what you shoot those things out of, they won't hold minute of paper plate at 100 yards.

Consistant cartridge cas length
Uniformity of cartridge cases
Consistant cartridge cas length

And the list goes on.

n2ishun said:

Nah.
A world class shooter in a world class rifle cannot compensate for an inaccurate bullet.
Just try and get a sub MOA group from 62 grain green tips some time.
You *might* get lucky 1 group out of 1000.
I've seen some pretty shit bullets over the years.....still have a few boxes of late 50's, early 60's Nosler Partition 30 cal 150 grainers that are machined, visible tool marks.
I don't care what you shoot those things out of, they won't hold minute of paper plate at 100 yards.

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It was a joke. Partly.

"What makes a bullet accurate?"

Accuracy is: when you aim exactly at X, all your bullets land in exactly the same place, relative to X. What can go wrong? A bullet moves through air so there are aerodynamic forces. Drag slows the bullet, drag force changes with velocity. If that air is moving across the bullet path - up, down, or sideways - lifting forces are generated that deflect the bullet. Rotation introduces forces that deflect the bullet, different effect if the rifling cuts deep. Gyroscopic forces try to keep the bullet rotating around an axis parallel to the rifle bore at the time of firing. A long ranges, the bullet axis of rotation may point in the direction of motion. Gravity causes the bullet to fly in a curve. The shooter has to adapt to that curve. If there was no air, there would be no drag so the velocity would not change. If there was not gravity, the bullet path would not be a curve.

Shape - if the shape is wrong, aerodynamic forces make the bullet nose sweep out a circle or even tumble - rotation is not exactly around the long axis. Wind speeds and angles change the size, orientation, and shape of the circle - maybe it will become an oval and impart lifting forces in different directions. A wind from X will generate a lifting force I, a wind from Y lifting force J - depending on the wind you see horizontal on the target. High-drag bullets react more strongly to wind speed and direction. This changes the flight time and you see vertical on the target. Keep in mind, most bullets are heavy in the back, not the front. That condition is important for stability.

Materials - in a perfect jacketed bullet, the jacket is soft enough to deform to fill the bore so gas does not cut the jacket, hard enough to retain its shape under internal ballistic acceleration, strong enough to hold together under the RPMs and prevent exploding bullets, and is perfectly uniform - one side is not thicker or heavier than another. During point-up, the jacket mass has to retain its uniform weight distribution and dimensions. The lead core must contain no bubbles, during seating it must deform uniformly into the jacket, it must have identical density throughout its mass, and deform uniformly when pointed up. In my experience, meplat is a drag factor but it doesn't matter at velocities around 3,000 fps. Non-uniformity has similar effects to "bad" shapes, that is rotation is not exactly around the long axis, lifting forces are created, flight time changes from shot to shot and accuracy suffers.

Barrel - for equal firing forces, barrel harmonic movements must be repeatable. The chamber and throat must be uniform and not introduce yaw into the bullet path or chew up the jacket. A chewed up jacket is not uniform. The rifling must be deep enough to make the bullet spin but not so deep as to weaken the jacket. The rifling cannot chew up the jacket. The crown must be uniform. I only have experience with free-floated barrels, except for the part about barrel harmonics, I do not know if it matters if the stock presses on the barrel. The barrel must be torqued into the action - you need a solid connection. Based on my experience, I cannot tell if there is such a thing as "too much" or "too little" torque. I use 30 ft pounds. Other people use more torque.

Given the ways and degrees that this can go wrong, I am regularly amazed by how accurate our equipment is and how fantastically good some people are at estimating the wind - especially compared to me.