[yarddog]

I don't know a whole lot about ballistics but assuming the bullets are not out of round, there are two things which contribute to stability.

The first is bullet construction. Unfortunately, the bullet which is most efficient in traveling through the air is one of the worst as far as stability. That is a pointed boat-tail.

This can be overcome by gyroscopic spinning, and it takes a fast twist to stabilize that type bullet. A tubular shaped bullet which is hollow point and hollow based is the most stable.

The reason those early 5.56 bullets would tumble after they hit was because of the bullet shape, they had a fast enough twis to stabilize the bullet but not when it hit something.

[hiredhand]

There is a fairly specific relationship between bullet weight, diameter, rifle twist rate, and it's ability to stabilize. You're correct about this. I've got a rifle now (a Rem M-700 .243) that can NOT stabilize bullets any heavier than 95gr! Actually, it's not that it can't, but the velocity which is required to cause stabilization can't be safely achieved! It would probably stress the rifle and create unsafe pressure conditions.

You're also correct about 5.56mm rounds tumbling because of striking something prior to their intended target! This WILL and DOES happen! The .243 would do this too if I used FMJs, but I use mostly 85gr HP/BTs and they fairly disintegrate if I strike something prior to the target. I like this because it's fairly flat where we hunt, and therefore somewhat safer.

As I remember, Eugene Stoner warned the U.S. Army Ordinance Dept. about changes that they made to "his" rifle (the "AR"). Among these were deminsional changes made to the throat of the chamber, resulting in excessive freebore, which with semi-autos is largely responsible for tumbling projectiles at a distance. :-)