Bullet stability

[Fred_C_Dobbs]

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.

[jo191145]

Thanks Fred, I did not know that. Still not sure I do but thats just fine too

In traditional Joe sixpack style (which I am) I ran the numbers for 66 and 70gn bullets through JBM.
Kept all the other variables the same.
It does show a considerable difference or at least more than I expected.
66gn = 1.003 SF
70gn = 1.064 SF

Good to know, Thanks.

[Bill K]

My Wild A$$ Guess would be they're not so much wobbling.
Merely flying out the barrel tips up and eventually straightening out.
I now and always retain the right to be totally wrong.
Odd either way.

Maybe I'll try blasting some into the snowbanks in my driveway.
Trigger finger sure is getting itchy this winter. Need to hear something go bang.

In an attempt to duplicate your velocity....
How much 10X are you using?

Know how you feel. Want to get the 223 and 204 out and fire some rounds also.. But this last week as been nothing but plowing and shoveling snow. Over four feet right now. Bill K

[jo191145]

Bill

The weather idiots say we have 84" already this season. The bulk of that was just in January and thanks to the arctic jet stream we had it refuses to melt. Had to pull those decorative snowshoes off the wall and figure out how they work
Some folks may be used to that much snow but its rare here in Ct. Was odd to walk out to the shooting range and not see the bench.
Things are getting better though.