Do heavy 5.56 bullets need more speed?

[Unclenick]

TL,

Greenhill's original short form did not include the 2800 fps constant change because, as Miller points out, it was all based on subsonic flow. Miller added the 2800 fps factor, AFAIK. In the March 2005 PS article, Miller says (with original formatting):

"The overturning moment, implicit in the denominators of eq A-C, is the only velocity-dependent term. A VERY crude approximation to correct for its velocity dependence is to multiply the calculated s by the 1/3 root of (v/2800) and calculated t by the 1/6 root of (v/2800), where v is the velocity in ft/sec."

So Miller wasn't impressed with its sophistication, either. However, in his July 2009 PS article about how accurate simplified approximations are, he's got comparisons, including to the late Robert L. McCoy's modern equations of stability, which show Miller's version it to work out to be the best of the simplified equations it's compared to. That article has the full collection of input data, so you can run the numbers by other estimators like the one at Kolbe's site, above.

In the 2009 article, Miller hints that more work was needed, as the current one apparently did not work well above 3600 fps nor with plastic tip bullets or hollow base bullets. I note the version on the JBM site simply clips the plastic nose off, presumably because it doesn't add enough angular momentum to coning motion to contribute a lot to the overturning moment, though its profile will contribute to drag, so some additional correction is present in that version. Unfortunately, Don Miller passed away in 2012, so we'll never find out what future refinements he may have had in mind.

Originally Posted by Shadow9mm

I did not realize the hornady bullet used a secant ogive. To me that explains a lot. Always heard secant was a lot more temperamental than tangent.

Yes. If you look at the secant ogive's abrupt change in angle, it can theoretically contact the throat rifling at its widest point, where a tangent ogive contacts it at the narrowest point, which allows cone and taper centering to occur that is absent in the secant case. In reality, there is some radius at the ogive's point of departure from the bearing surface (the shoulder), but it is steeper than the angle into the throat. So I think there is just too much wiggle available if the bullet isn't started into the throat as straight as is perfectly possible. This would make a good test project for this summer's shooting.

[tangolima]

Thanks unclenick. More to read during lunch.

-TL

Sent from my SM-N960U using Tapatalk

[Metal god]

"The overturning moment, implicit in the denominators of eq A-C, is the only velocity-dependent term. A VERY crude approximation to correct for its velocity dependence is to multiply the calculated s by the 1/3 root of (v/2800) and calculated t by the 1/6 root of (v/2800), where v is the velocity in ft/sec."

…

[Unclenick]

Yeah, but it is, after all, a form of rocket science, so this kind of thing will come up now and again among those of us who can't help drilling down beyond what the popular literature covers. For those not interested in that level of detail (probably most folks), just ignore it and take heart in the fact that knowing the math doesn't give us better trigger control or hold consistency.

[Metal god]

I have no choice but to ignore it , that is way above my knowledge and understanding . I’m still working on ( time doesn’t exist )

[Shadow9mm]

Originally Posted by Metal god

I have no choice but to ignore it , that is way above my knowledge and understanding . I’m still working on ( time doesn’t exist )

Time exists, but i still don't know if it's something that was created (a construct of the mind), or a dimension in space....