Re: Bullet stability
Posted: Wed Feb 23, 2011 11:40 am
jo191145, the old Greenhill formula used a specific gravity of 10.9 for lead. According to Lilja, modern jacketed bullets run from a specific gravity low of 10.0 for JHPs to a high of 10.4 for soft-nosed bullets, and that includes the copper. Polymer tips float so their SG is less than 1.0, probably a lot less. So when you include the length of the polymer tip, you are causing the calculator to overestimate the mass contained in that portion of the bullet by more than 1000%. The only way it can rectify that discrepancy is to presume a higher ogive number (longer, more tapered) or a longer boattail or both. Either change amounts to concentrating mass closer to the axis of rotation, which lessens the rigidity of rotation, which reduces stability.
From Long Range Hunting:
"...If your (sic) comparing a bullet with a plastic tip or a large void behind the meplat jacket where there is no lead core present, this can change the transverse inertia moment and transverse radii of gyration quite considerably and therefore the approximation given by the miller formula, underestimates the stability by an appreciable margin."
FWIW, in Applied Ballistics, Bryan Litz also mentions that the Miller formula (on which the JBM calculator is based) is accurate only to +/-10% because of its dependence on SWAGing information that is beyond the capabilities of Joe Sixpack to provide, like precise details concerning the shape of the bullet.
From Long Range Hunting:
"...If your (sic) comparing a bullet with a plastic tip or a large void behind the meplat jacket where there is no lead core present, this can change the transverse inertia moment and transverse radii of gyration quite considerably and therefore the approximation given by the miller formula, underestimates the stability by an appreciable margin."
FWIW, in Applied Ballistics, Bryan Litz also mentions that the Miller formula (on which the JBM calculator is based) is accurate only to +/-10% because of its dependence on SWAGing information that is beyond the capabilities of Joe Sixpack to provide, like precise details concerning the shape of the bullet.