A few weeks ago, I covered the myths about bullet stabilization in the post "Stabilization Mythology", but I didn't touch half of one of the biggest myths about 5.56nato, tumbling bullets. Specifically, I covered the bullets in flight, but not what happens in tissue (the other half of the tumbling myth).
Well, James Rummel addressed the tumbling bullet issue over at Hell in a Handbasket; and also linked to my original post. James is a good guy, and he's right about the end results, but like most folks he's shaky on the science and engineering of why.
Soooo... what about tumbling bullets? I mentioned in the other post that the myth was, the original M16 barrels and ammunition were made deliberately unstable so they would hit the target "tumbling".
What I didn't mention, was the second half of the myth where the "tumbling bullet creates massive explosive wounds tearing up flesh so bad that you could hit a guy in the leg and blow it clean off" ... or at least that was the myth in Viet Nam. The claims have moderated somewhat since then, but you still hear people saying "the bullet tumbles through and shreds everything in its path" and other such garbage.
This, like the other myths about bullet stability, is also very untrue. As I described above, a bullet that is tumbling in flight is likely not to hit the target at all, and it's terminal effects will be entirely unpredictable and inconsistent.
The stabilization in air of a bullet has very little effect on it's terminal performance; except that a bullet which is unstable in air, will generally become more unstable in tissue. Bullets that are stable in air may "tumble" in tissue, or they may not; and "tumbling" itself is generally a mis-statement of what happens.
Important to this discussion, referring back to the original post; rifling twist rate doesn't significantly effect the stability of the bullet once it enters tissue.
The twist of the rifling, the bullet length, the bullet weight, and the bullet profile all effect stability in flight. Once you hit a dense fluid (and effectively, tissue is a dense fluid), the forces of hydrodynamic pressure and inertia are so great, that the twist of the rifling (and it's primary stabilizing component, gyroscopic forces) is nearly irrelevant.
I'm not going to deeply address the terminal performance of bullets in tissue here, but I do want to talk for just about tumbling, and the wounding mechanism of the 5.56 nato round.
Mostly when people talk about tumbling bullets inside tissue, what they picture in their mind is a bullet pinwheeling like a buzz saw through a body. This image is generally incorrect.
Generally speaking, when a long spitzer bullet hits tissue (long relative to a pistol bullet or a round ball), the center of pressure of the bullet will radically shift forward, to approximately 30% of the length of the ogive behind the point of the bullet; which for most bullets will be very far in front of the center of gravity (and thus the majority of the mass). Because of this, the base of the bullet will tend to rotate around the center of pressure, past the nose once, or at most twice (due to inertia and the initial shock of the wound); and then the bullet will travel base forward (being led by it's mass) until it comes to rest, or exits the body (when it generally WILL completely destabilize and actually tumble). If the bullet travels far enough in tissue (somewhere between 14" and 20" depending on the exact bullet, and the density of the tissue), it will tend to rotate back to point forward, and then to base forward again.
That's not exactly tumbling; and it's certainly not "cutting through flesh like a buzz saw", or anything like it.
This rotation, or "yaw" does produce a significantly larger permanent wound cavity; unless the bullet disintegrates before achieving sufficient penetration to reach vital organs and blood vessels. The problem being of course, that the yaw reduces penetration; and often DOES cause disintegration before that minimum level of penetration is achieved (especially with lighter bullets).
Now, disintegration isn't necessarily a bad thing; so long as there is sufficient penetration, and the fragments created have enough mass and velocity to continue producing significant penetrating trauma (something that is again a problem with lighter bullets).
This is why the 55gr and 62gr 5.56 nato rounds produce significant disabling wounds when they strike their targets at about 2500-2700fps or above. At that velocity they tend to penetrate 7-10" within a body before yawing to a vertical position, at which point the unbalanced forces on the bullets tend to break them apart at the cannelure, into two or three large, and several smaller fragments; with the fragments typically penetrating another 2-4", creating wound tracks like a small buckshot shell (or, with very light bullets, more like birdshot).
Unfortunately, at velocities below 2500fps, both bullets will either tend to break up before achieving significant penetration; or they will tend to simply pass straight through a human body (at least a thin one anyway, and most of our enemies aren't exactly overfed), leaving a very small wound channel, and creating little shock; unless they directly hit a major organ, blood vessel, or bone. If they don't disintegrate, the bullets will generally still pass end over end once, or even twice; but will not break up, and will usually exit the body base first.
With the standard 20" M16A1-A3 barrel, this isn't really a problem, because the 55gr and 62gr nato loads will maintain 2500fps out to around 200m, but every inch you cut off the barrel length cuts something like 25m off the effective range of the loading.
The M4 carbine has either an 11.5" or 14.5" barrel (depending on what generation M4 it is. Most issued today have 14.5" barrels). A 14.5" barrel will produce between 200fps and 400fps less muzzle velocity (yes, it really is that variable depending on loading) than a 20" barrel; which translates into a HUGE difference in downrange performance. The range at which the 62gr load will reliably stay above 2500fps from the 11.5" barrel, is about 80m; from the 14.5" barrel it's about 125m.
Now, with good shot placement, those through and through wounds will probably kill your target just as dead; but they'll do it a lot slower, and they won't necessarily slow them down all that much in the mean time. The disintegrated bullets wounds CAN be very nasty and immediately disabling, but if they didn't penetrate far enough, or they penetrated in a poor location, they aren't consistent either.
This means that we don't have much consistency in stopping power; and what consistency we have is inside 125m. Yes, the 5.56n round in the 55gr and 62gr loadings CAN be effective at 300m, or even 600m, but it won't be consistently effective at those ranges, or any range beyond about 125m (or even less).
This is the root of the ammo controversy for the M16/AR platform. Since we can't use an expanding bullet (Hague convention), we are limited to using a disintegrating bullet; or to changing our wounding mechanism entirely and using a much heavier bullet which will produce a greater wounding effect without having to disintegrate.
This is why 68, 72, 75, and 77gr loads have been used by special operations forces for some time now (as the mk262 and other loadings); and why 6.8mm and 6.5mm alternatives are being so highly touted.
The longer, heavier bullets are more ballistically efficient, and more effective at longer ranges (incidentally, they also tend to expand; but because that expansion isn't deliberately engineered into the bullet as a wounding mechanism, that's OK under the Hague convention. No, it doesn't make any sense, but that's the way it is).
Additionally, they have greater penetration at all ranges and velocities, without disintegrating; but they are less likely to overpenetrate as well; because rather than disintegrate or pass straight through, they tend to yaw in the body and move off track, creating a larger and longer wound channel.
The 55gr or 62gr 5.56 bullet MAY, if they disintegrate properly, produce a more damaging wound; but that disintegration and its results are not consistent in their wounding effect, whereas the effect of the heavier bullets is relatively consistent and predictable.
...but... none of that wounding capability, is from "tumbling"...