What makes a bullet accurate?

[VargmatII]

So, for the better part of this weekend I have been pondering the question, what makes a bullet accurate? Is there one or more rule of thumb kind of design aspects or is it all trial and error?

The thing is that I am currently testing a CNC turned bullet made from brass alloy as sort of a Beta tester. I have shot just over 100 bullets over the last 3 weeks and it has rasied some questions.
As most of us, I am used to matchbullets, such as Sierra Matchkings, Lapua Scenar and Hornadys various types of matchbullets. Most of them are easy and sometimes very easy to get to shoot sub MOA or better. In this case its the tried and tested 308 winchester that I am running, but I have had similar experiences with 6.5 caliber bullets from the same makes.

So the thing with the CNCd alloy bullets is that they seem to be very picky on the charge weight. I have tried multiple times and in a regular ladder you can get the following numbers:

41.1 grain, 2" spread.
41.2 grain, 2,6" spread.
41.3 grain, 0.75" spread.
41.4 grain. 1,7" spread.

This is with VV N140 powder and its all shot at 100 meters range. So it seems that you either are exactly on a node (if such a thing exists) or you are completely off. I am sure that if I did the same thing with my regular load of SMK or Scenar L 175 grain I could probably be atleast +/- 0.3 grain or more and it would barely show up on target, especially not on 100 meters.

So this got me thinking, is there something in the design of the bullet that is off? Something that the manufacturor should reconsider?
Is there a sertain angle of the boat tail, something else that could cause the bullet to be more picky than it should be?

I know I could be more detailed but Im curious if this raises some thoughts around here?
I know my shooting can be questioned but I am using a tried and tested matchrifle, all shots are from prone and I am using a Ckyepod and rear bag. I usually have no problems at all keeping my groups sub moa or better at 100meters.

 

[Hondo64d]

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

 

[VargmatII]

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

Im not an engineer either, just an avid shooter and used to shoot a lot of competitions.
The bullets are stable, that I am sure of. They dont seem to foul the barrel too bad either, I cleand it and have kept an eye on it during this testing.
I do however have a heavier version of the same bullet and I am worried that it wont stable in my standard 1:11 twist barrel. For as you write, mono bullets are longer than their regular counterpart.

I have shot 3 shot groups when doing initial ladder tests. The better stages in those ladders has then been done again with 5 and 10 shot groups for additional test, chronographing and confirmation.
I do however agree on that it is a low count from a statistical standpoint. But then again, I have never needed much more than a ladder test or two to determine what chargeweight I want to run with Scenars or Matchkings. So there is that.

The easiest ladder test I ever did was with Lapua 139 grain Scenars in a 6.5 Swede. The whole ladder shot great and it was only a question of picking speed. So I picked one of the top ones and kept running that until the barrel was shot out. Shot the tightest group I have ever shot with that one, at 0.23" C/C. Never was able to to that again...

 

[bax]

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.

 

[VargmatII]

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.

I dont know where the design came from. The company has had a .223 monometall bullet in the market for about 2 years. They are going to produce 308 and 6.5 caliber bullets in the future and thats where I came in. It may be that the 308 caliber bullets that I have on hand is nothing more than a scaled up version of their .223 design, I really dont know.

The thing that strikes me about the mono-bullets are the shape of the boattail, which is nothing like an SMK or Scenar bullet. I have asked the manufacturer about this and maybe it will lead to another design to try out.
The way I figure it is that if they cut a different angle of the boattail, this will also reduce the weight of the bullet and more than likely change the balance (COG) of it. That may be detrimental, or maybe it will make them more forgiving than I have found yet.

 

[n2ishun]

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.

 

[secondofangle2]

The shooter

 

[AK35Whelen]

The shooter

This.

 

[n2ishun]

The shooter

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.

 

[VargmatII]

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.

And the vast majority of us are happy to pay top money for matchbullets of various kinds.
If it was all up to the shooter none of them would sell.

 

[Dough300]

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

And the list goes on.

 

[secondofangle2]

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.

It was a joke. Partly.

 

[bax]

"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.