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Er...about the M-16 "tumbling" rounds. This is more commonly called a "keyhole" strike, because of the keyhole shape produced on the a target as a result of the bullet tumbling during flight. NO rifle is supposed to produce keyhole strikes!

The reason the M-16 (especially the older ones) did this is because they were manufactured with excessive freebore, in order to facilitate a wide variance of manufacture's ammo. Excessive freebore (ESPECIALLY in semiauto rifles) causes the projectile to contact the lands of a barrel on a NON-CONCENTRIC axis. This causes the same effect as when you throw a football, and it goes all "wobbly". The bullet exits the bore on a less than perfect axis, and by the time it's travelled a couple of hundred yards (sometimes less), it's in a full fledged tumble.

It's a VERY bad thing to have happen as it destroys accuracy at longer distances, and renders an otherwise good weapon, ineffective and unsuitable even under good shooting conditions!

The newer A-2s, and A-3s aren't supposed to keyhole rounds. We've got a couple of Armalite A2s and they don't keyhole, but when I was active duty we used A1s that did.

My biggest gripe about our ARs is that the mechanics in their uppers are needlessly complex, and that it "craps where it eats". The tilting block, and gas piston/tube mechanism is widely used (except for with things like H&Ks blowback system), and I can't for the life of me figure out why they settled on the complex little system that the AR uses.

...having now complained about it I have to admit that a buddy of mine in an operational unit just returning from Iraq tell me that their A2s and M4s were VERY reliable and rarely malfunctioned.

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.

about the M-16 "tumbling" rounds. This is more commonly called a "keyhole" strike, because of the keyhole shape produced on the a target as a result of the bullet tumbling during flight. NO rifle is supposed to produce keyhole strikes!

The marginal spin was supposed to give enough stability that the bullet was stable in flight, but would tumble AFTER entering flesh