To renew the alloy/bullet design subject. Go back to the 'marshmellow' down the barrel example. Assume the bullet does NOT fit throat/freebore/bore perfectly. No case/neck for this discussion. Push on the base and it must expand into all empty space (incompressible alloy) , nose isn't moving yet. Wall friction keeps the 'skin' stationary for a while. Center (core) FLOWS toward the nose, expanding the nose possibly into the grooves. Alloy FLOWS from throat into bore (reduced diam.). Nose begins to move. Nose & body start to spin but not at the same rotational speed until body is entirely into the rifling. The soft alloy has lots of stress (pressure) and strain (movement) but as it's 'gooey', so no fracture. As to design, the longer the bullet (length to bore ratio), the more differential in inertia, base moves before nose. Initial engraving increases the differential. All this happens quickly but get a good visual/mental picture of what is happening.
Now, harden the alloy some. What changes? Nothing! The alloy just FLOWS less easily/rapidly! What can occur? It is assumed flow occurs geometrically perfect (proper throat/freebore/bore). We also assume that the alloy CAN flow and NOT create stresses that will distort the bullet. The highest bullet acceleration occurs when it is just in the bore ( directly related to base pressure) therefore most of the groove spin force occurs there. Note: velocity is the summation of acceleration and time. So now we want minimal flow (distortion). So far we haven't spoken of lube or friction. Friction coefficient is related to the materials at the junction. Lube reduces the 'effective' coefficient. Actual friction is determined by the force applied at the junction. Difference in friction around the bullet can cause distortion. Lube grooves. Strength of a rod is determined by material strength and diameter - so - grooves weaken the bullet. Empty grooves weaken the compression resistance of the grooves. So shallow grooves (with angular or rounded shape) makes a stronger bullet.
Nose shape? Why not just flat (wad cutter)? BC - aerodynamics! But remember rod strength from above. Bore rider? Lots of metal in the bore that doesn't have to flow from the throat but if too strong it won't flow into the grooves. So we want an alloy that will flow through the irregularities of the entrance and then NOT flow. Un-attainium! Best we can do is experiment with alloy and real powder pressure curves (burn rate).
End of physics lesson.
As we are talking HV mostly, let's talk GCs. Interestingly pressure is the same on all surfaces of a closed container. If the base is not square with the bore, there is a radial force on the base which can cause the body to be forced to one side of the bore. Additionally, the GC must go through the 'flow' process with no distortion or upsetting the alloy flow. IMHO, annealed GC eliminated forming stress, softens the Cu closer to alloy and allows better flow.
Now, harden the alloy some. What changes? Nothing! The alloy just FLOWS less easily/rapidly! What can occur? It is assumed flow occurs geometrically perfect (proper throat/freebore/bore). We also assume that the alloy CAN flow and NOT create stresses that will distort the bullet. The highest bullet acceleration occurs when it is just in the bore ( directly related to base pressure) therefore most of the groove spin force occurs there. Note: velocity is the summation of acceleration and time. So now we want minimal flow (distortion). So far we haven't spoken of lube or friction. Friction coefficient is related to the materials at the junction. Lube reduces the 'effective' coefficient. Actual friction is determined by the force applied at the junction. Difference in friction around the bullet can cause distortion. Lube grooves. Strength of a rod is determined by material strength and diameter - so - grooves weaken the bullet. Empty grooves weaken the compression resistance of the grooves. So shallow grooves (with angular or rounded shape) makes a stronger bullet.
Nose shape? Why not just flat (wad cutter)? BC - aerodynamics! But remember rod strength from above. Bore rider? Lots of metal in the bore that doesn't have to flow from the throat but if too strong it won't flow into the grooves. So we want an alloy that will flow through the irregularities of the entrance and then NOT flow. Un-attainium! Best we can do is experiment with alloy and real powder pressure curves (burn rate).
End of physics lesson.
As we are talking HV mostly, let's talk GCs. Interestingly pressure is the same on all surfaces of a closed container. If the base is not square with the bore, there is a radial force on the base which can cause the body to be forced to one side of the bore. Additionally, the GC must go through the 'flow' process with no distortion or upsetting the alloy flow. IMHO, annealed GC eliminated forming stress, softens the Cu closer to alloy and allows better flow.