Jerry,
We have to clarify some definitions here.
When referring to bullets, static balance is not front to rear, but radial. It's an offset of the center of gravity from the center axis of the bullet. Static imbalance is typically introduced in bullets from jacket eccentricity (jacket is thicker on one side than the other, thereby causing the core to be off-center). This is why jackets with low run-out are so critical to precision. A small amount of static imbalance (0.0001") will make it impossible to shoot BR winning groups. Dispersion from static imbalance is related to twist rate, which is why many BR shooters choose the slowest twist possible.
A football has static imbalance due to the weight of the laces on one side.
Dynamic imbalance is harder to explain/envision. In engineering terms, we say that bullets have a geometric axis (simply the line defined by the bullets shape) as well as a principle axis of inertia which is determined by the mass distribution within the bullet. These two axis are the same if the bullet is perfectly balanced. If these two axis are not parallel, the bullet has 'dynamic imbalance'. If the two axis are parallel but not together, then the bullet has static imbalance.
Back to football, when you see a ball thrown that is 'wobbly', that perceived 'wobbly-ness' is what dynamic imbalance looks like. Note this analogy isn't strictly accurate, because the ball itself isn't dynamically unbalanced, it just wobbles because of being thrown imperfectly.
You could introduce a dynamic imbalance to a bullet by drilling a 0.030" deep hole (for example) 1/4" behind the CG on the left side of a bullet, and another hole of equal depth 1/4" ahead of the CG on the right side. This wouldn't necessarily ruin the static imbalance of the bullet, but would introduce a dynamic imbalance.
It is possible for a bullet to have purely static imbalance, or purely dynamic imbalance. In reality, every bullet has some amount of both, even if it's not measurable (because nothing man-made is 'perfect')
Static imbalance is the big enemy of precision. When the bullet is in the barrel, it's confined to spin around it's geometric axis. But when it emerges from the confines of the muzzle, it immediately begins spinning around it's principle axis of inertia (CG) (because it's now in free-flight). For a bullet having no static imbalance, it will come out of the barrel and fly straight. But if there is static imbalance, the bullet will gain some lateral velocity as it exits the muzzle.
Your question referred to static imbalance being related to the fore-aft position of the CG, which, for a bullet spinning on it's proper (long) axis, wouldn't have an effect on balance.
The fore-aft location of the CG does affect the bullets *stability*, which is entirely different from balance. A bullet can be stable and un-balanced. It's also possible for a bullet to be balanced and unstable.
Balance is a property of the bullet itself. Stability is determined/affected by it's spin rate, velocity, air density, as well as it's mass and aerodynamic properties.
On the subject of precision, stability has very little effect until you fall below the critical stability factor where the bullets don't fly point on and start key-holing on target. Even keyholing bullets can be relatively precise. I shot a 300 yard group with 300 grain solids from a .338 LM, 1:10" twist which printed ~1" long keyholes and the group was under 1 MOA. This is not typical or desirable, but just shows that stability and precision aren't necessarily linked.
For stability levels below 1.5, you can measure a decrease in the bullets effective BC due to the bullet flying with greater pitching/yawing angles, but again, groups don't suffer from this marginal stability. In fact, if you're dealing with bullets that have a great deal of static imbalance, you can expect *better* groups from marginally stabilized bullets because the slower spin rate results in less dispersion from the same static imbalance. Again, consider the tendency of short range BR shooters to choose slow twist barrels. Typically, stability factors of short range BR shooters are in the 1.1 range and it's not uncommon to see minor key-holing if conditions (air density, MV) are not favorable to stability on any given day.
Donovan,
To your questions about core height, I'll attempt an explanation, but admit I may be going out on a limb. I welcome comments from someone with more hands on experience actually 'making' bullets to comment on my statements here.
The question: Might it be favorable to go for a given core height vs. overall bullet weight when dealing with different lots of raw materials.
I *think* this may have to do with the array of tools available to the bullet maker; in particular core seating punches. Since jackets are tapered, and will vary in their thickness vs. height from lot-to-lot, the bullet maker is compelled to use different diameter core seat punches for different lots of material if he wishes to achieve the proper fit to jacket *and* make all lots of bullets the same weight.
For example, if a bullet maker only has one core seating punch, he may have to make the cores longer or shorter in order to achieve the proper fit to jacket and core seating pressure with any given lot of components. As you described, this would result in bullets of slightly different weight.
However, if a bullet maker has many core seating punches graduated in 0.0001" increments, he can accommodate the range of raw materials, and properly seat a core to any length necessary to achieve the desired weight.
The core seating punches used by Sierra for their MatchKing line of bullets has a clever feature. The face of the core seating punch is hollowed out, which allows the lead to relieve up into the cavity as much as it has to. This way they can use the same core seat punch on many different lots of lead and jackets, and there is someplace for the 'remainder' to go. Another function served by this cavity is that it acts to relieve and equalize the pressure on each stroke of the machine. For machines like those used by Sierra in which several forming operations are performed on each stroke, it's important that the energy of each stroke is equally distributed.
Berger's presses are single stage, meaning that the energy from each stroke is used on one forming operation at a time. The correct core seating punch is selected for the given lot of materials such that everything 'fits', and results in bullets of the desired weight.
Both process can make good bullets. In the end, the single biggest thing affecting precision is the jacket run-out. Over the years, Berger has been able to maintain a reputation for high precision. This has been achieved primarily thru a strict commitment to using jackets with 0.0003" run-out or less (which makes bullets balanced to within 0.0001"). Other advancements in bullet making have been made related to manufacture-ability, and lot-to-lot consistency. But the biggest thing in precision bullets remains static balance, and it all starts with concentric jackets.
Jackets with 0.0003" run-out or less has set Berger Bullets apart from others since the beginning. Walt Berger shot is way into the BR Hall of Fame with bullets that were made on jackets that he turned on a jewlers lathe, one at a time, so they had less run-out than any other bullets. Eventually J4 started mass producing the high precision jackets, and Berger has used them ever since.
I'll let Eric Stecker know about this thread and see if he's able to address any of my comments on core seating punches.
Take care,
-Bryan