Outsider, with no experience to speak of,
Just an engineer with delusions of competence.
If we assume, for the moment, that the reason for the observed phenomenon that cast bullets tend to be less accurate at high velocities than jacketed bullets of similar weight and shape is that the tangential velocity of the bullet causes instability in flight, as the rotational forces act on an unequal mass distribution, then we still need to find out why this seems to affect cast bullets more than jacketed, and from that, derive correction measures so that we can shoot our (cheaper) cast bullets at the same velocity and accuracy as the more expensive jacketed ones. Otherwise, why bother in the first place?
Since RPM of the bullet is a linear function of twist rate (revolutions per unit distance) and velocity (distance per unit time), or, mathematically, r/d * d/t = r/t, it is clear that the issue is not the RPMs per se, but rather the bullet's reaction to those forces. So why is a jacketed bullet more able to resist those forces?
I'll divide this into two sections. First, differences between the ways the two bullets are made, and second, differences in the ways they react to the stresses of firing.
Making bullets: Cast bullets are, well... cast. Liquid metal is pured into a mold, and allowed to solidify within that shape, with no external pressure applied. As I understand it, jacketed bullets are cold-formed under pressure from solid (extruded?) lead and thin-sheet copper. The pressure of forming would tend to reduce uneven distributions of material, and would also likely drive any remaining inconsistencies toward the center of the bullet, and thus closer to the axis of rotation, thus reducing the force acting upon them at a given RPM. BTW, if that's true, then a smaller-diameter cast bullet should be more accurate at a given RPM than a larger one (assume the same BC, SD, and defect rate), since all defects would be experience less force. Has this been demonstrated?
So to control this aspect, we would need to make more internally consistent bullets. Can this be done by changing alloys for more consistent mold fill, somehow improving the evenness of the alloy mix (to get a more even density distribution through the bullet), or anything else that the caster can do, or is some sort of pressurized mold design needed? I'm picturing in my head a mold with a slightly deeper than final sized cavity, and some sort of plunger arrangement on the sprue plate.
Shooting issues: This seems to me to be primarily a surface hardness issue. Copper is much harder than lead, so if a copper-jacketed bullet and a lead bullet (swaged or cast) are shot through the same barrel at the same speed, the jacketed one will emerge from the barrel less deformed (and thus closer to it's original mass distribution) than the lead one. If this is what's happening, then there should be a clearly measurable improvement in accuracy at high RPMs from a harder bullet. And it would seem that those changes should be nearly linear, unless hardness measurement scales are not.
--Shannon
If the RPM idea is correct,
Quote:
Originally Posted by
felix
Shannon, correct on all accounts. Yes, 22's shoot best. But, is twist the culprit, or is it lower mass, or what? ... felix
Then it would simply be that the imperfections that exist within the bullet are closer to the axis of rotation, and so experience less force, thus disturbing the bullet's flight less than if the bullet was larger.
If we also assume that the "defect density" (defined arbitrarily by me as the number of deviations from perfect uniformity per unit volume of bullet) is constant and also assume that the distribution of defects throughout the bullet is random with respect to position, then as caliber goes up, the "RPM threshold" should go down, as proportionally more defects will be farther away from the axis of rotation.
In other words, if we hold BC (essentially a numerical abstraction of shape, as I understand it) and SD constant (thereby keeping a constant mass/diameter ratio), then as we went from .224 to .308 to .457, the bullets will show reduced accuracy at lower RPMs.
Centripetal acceleration is a quadratic function of angular velocity and a linear function of radius, if the deflection from the bore axis is entirely due to such accelerations, we should see the following proportionalities, with the same assumptions as above:
The deflection will increase as the square of the change in RPM
and
The deflection will increase directly with the change in bullet diameter.
Or, doubling the RPM will cause bullets of the same size to shoot a four times larger group, while doubling the diameter will cause bullets with the same RPM to shoot a two times larger group. Since bullet RPM = twist rate (turns per inch) X velocity (inches per second), you can substitute velocity for RPM directly without changing the proportionality.
Is this true in the real world?
-Shannon