Rifle bullet stability

[PJ Hardtack]

This topic may be better suited to the 'Reloading' column, but Spencer shooters seem to concerned about the problem with their repros. The current "Rifle Magazine" has an article on bullet stability and how it affects accuracy.

It's long been a mystery, even to rifle makers, as to why some twists stabilize bullets of various weights, types, shapes. It hasn't been long that they seem to have gotten a handle on it. Up to now, common knowledge has been that a heavier bullet requires a faster twist to stabilize.
My Shiloh 50-70 carbine is a case in point - it has the current 1x26" twist and stabilizes 540 gr, 450 and (much to my delight), 375 gr. bullets, as the heavy bullets give heavier recoil.  I haven't printed it on paper past 100 yds as I regard it as a 100 yard-max hunting carbine. In my Shiloh 50-70 rifle, with the older 1x36" twist, bullets heavier than 450 grs keyhole at 50 yds.

Theoretically, the 375 gr.should be "over stabilized" by the 1x26" twist, but there are other factors at play - bullet stability (centre of gravity), determined by rate of spin and the centre of pressure, the force of the atmosphere on the front of the bullet. Velocity plays a role, but not as great as you might think.
If you're the type that sits up nights worrying about this stuff, you can buy a Juenke Bullet Concentricity Comparator, download the Sierra ballistic program, get the Bryan Litz CD on the topic and the original 1879 Greenhill Formula.

The latter is generally concerned with establishing the ideal  bullet weight as the determining factor. For example, I long ago used it to establish that the ideal bullet weights for .30 calibre was 165/168 and 150 gr. for. .277 - in MY rifles. It's a happy combination of bullet weight, bullet stability and rate of twist. Your results may vary, like they say .....
How do I explain the fact that lighter weight bullets also shoot well in these rifles? Re-read what I wrote above. My heavy barrel Howa .308 'Tacticool' groups 110 gr bullets like a champ and my .270 groups 130/140/150 bullets in a the same group at 100 yds, at least good enough for hunting purposes without re-zeroing.

A lot of myths are out there, based on the poor results of some bullet weights in various calibres, but modern bullet makers are dispelling those myths with improved bullet designs. Up to now, it's been a hit or miss (pardon the pun) proposition, finding out through experimentation, which bullets of which weight work in the twist of a particular rifle.

A lot of this ballistic theory is way over my head as mathematics was never a strong suit for me. I think I'm getting a headache ..... ;>(

[Herbert]

Stability of bullet flite has been understood for well over a hundred years (every now and then companys try to reinvent the weal only to find out that the round weal realy does wok the best).The factors are rate of twist of the barrel,groove diameter of the barrel,bullet lenth(weaight is a by product of bullet shape,a large hollow point can be longer for the same weaight) and velocity.If you have a barrel with a certain twist and groove diameter,you adjust the othe factors to that,there is a point were it is impractical to find the right combination for a certain caliber,the 56-50 Spencer cartridges has to be a certain OAL so there is very little room to adjust the bullet lenth and velocity if the twist and groove diameter is wrong

[PJ Hardtack]

Sorry I mentioned it .....

Perhaps a read of the article followed by an email to John Barsness, the author of the article, might be in order. Like me, he was impressed by the latest research information on the topic which de-mystified the science.