Depends which round you're talking about. M855, despite the cries of its critics, performs pretty much exactly the same as M193, but with longer range. I will link to the study later, but M193 tumbles at almost any velocity, and fragments most consistently over 2300 ft/s. This threshold is determined by the jacket thickness. M855 has the same jacket thickness as M193, so it should fragment at the same velocity. You could reduce the thickness of M855's jacket and improve its performance, at the cost of penetration.
Mk. 262 has a thinner jacket than either M193 or M855. It typically tumbles like an FMJ and fragments. Like M193, it will tumble readily at almost any velocity. It fragments most consistently at velocities over 1,800 ft/s.
Mk. 318 definitely does not expand and certainly does not mushroom. Definitely. It is not a hollowpoint.
There was a pretty good study that came out a while back that showed that M855 does sometimes have trouble tumbling. Many authors have used the equations for stability in media to show that this is not due to the twist rate, but that's not the whole picture. I believe this is actually do to the twist rate in a roundabout way. The twist rate determines the stability through media, in this case air. A tighter twist rate will better stabilize the projectile, reducing the precession of the bullet (the degree to which it deviates axially around the flight path). It is this reduced angular deviation that can cause through-and-through wounds, not the bullet being stable through flesh. In other words, a bullet stabilized by a 1-in-7 twist rate barrel may hit the target at a shallower angle and thus yaw later than one stabilized by a 1-in-9 twist rate barrel. I highly suspect this is why you will be hard pressed to find a gel test video online of M855 being fired from a 1-in-9 twist rate barrel and failing to upset within about the first 5".
Somewhat paradoxically, this tight twist rate should give M855 exceptionally consistent long-range terminal effectiveness (though whether it's exceptionally effective remains to be seen). The same excellent stabilization that minimized precession also ties the bullet more closely to its original orientation through its flight. That means that at long range the bullet is flying through the air at an upward angle relative to the arc of its flight. If it hits a target at this angle, it should upset readily and tumble as designed.
I have sources for everything in this post, but I can't get at them right now. I will edit this post later with the relevant information.
But we have the above credible study that shows that M855 doesn't fragment well beyond 200 meters. How do the other notional cartridges perform? Similarly? If so then there's a problem.
I mentioned handgun and hunting ammo for a reason above. We're talking wound profiles here in part (intermediate ballistics too). If you're not causing critical wounds on Pathan tribes men (obtuse reference to malnourished children not withstanding) where are you causing the critical wounds? Where is the larger permanent wound cavity when talking about one of these fellows? Is it in the vitals or in their back?
If it takes more than one shot to put one down at range, how does that affect your weight savings? If it takes 2-3 or even 4 rounds of 5.56 are you saving any weight over a larger caliber? Especially if you're talking about intermittent chances at hits due to cover and such?
E5M, how many shots of 7.62 ball or .338 did it take to put a Haji down? One? More than one?
E5M Would you rather carry fewer rounds of ammo that'll reliably put targets down or more rounds of ammo that takes several rounds to put a Haji down?
I've said as much, if you use a ballistic calculator on some velocity averages I've shown. M855 does not tend to fragment beyond 200-250m. Neither does M193. Neither does Mk. 262. Neither does 6.8 SPC (I am actually uncertain that 6.8 SPC FMJs fragment at any range, as the muzzle velocity is very close to the fragmentation velocity for M855, and I think the jacket is thicker). A lead-free GPC would most likely not fragment at any range.
Re: Sebastian's comment, M855 loses approximately 100-150 fps in the M4 vs. the M16. Given a minimum fragmentation velocity of 2300 ft/s (it can be as high as 2600 ft/s or as low as 2,000 ft/s; there's a huge amount of variance in the construction of SS109 clones), when fired from the M4 M855 will fragment at up to 225m. Assuming a shot out barrel and somewhat underloaded ammunition, this can drop to as low as 150m, not 50m as you quoted. It does not make sense for M855's fragmentation range to fall so drastically from short barrels; it simply doesn't lose enough velocity vs. the M16.
The problem here is not the perception that 5.56's terminal effectiveness leaves something to be desired, it's that you can't get a drastic improvement by doing fundamentally the same thing. Rifle wounds are severe because of their velocity and energy, and they lose both of these things at range. All rifles do. Criticizing 5.56 for not tumbling quickly enough, or not fragmenting past a certain range, or for going through a target without upsetting is off-base when what you're proposing is replacing it with a cartridge that will have all of these same problems, while pinning your hopes on forty thousandths of an inch.
7.62x39 often causes through and through wounds that can be easily treated. 7.62 NATO is also known to do this. All these cartridges are, on a fundamental level, similar. A 'roided up Grendel will not give magical results, even if it gives good performance.
...But it may not have to. What's most important is penetration, and 5.56 does seem capable of doing that job, most of the time. Remember, at the turn of the last century, it was considered perfectly acceptable to be striking targets at the maximum ballistic range of a rifle, with less energy than many handguns of the time. The bullets penetrated, and that was enough. Likewise, even individual artillery fragments are relied upon to cause injury to enemy soldiers, when they have pitiful amounts of energy even at fairly close range.
Given that riflemen do not cause most of the casualties in warfare, given that penetration is probably the most important factor terminal ballistics, and given that 5.56 has ample ballistic range and penetration of tissue, I think it does the job fairly well, and it's light, to boot. Could this change? Of course. Has it? I remain unconvinced.
Edited by BabyOlifant, 04 December 2013 - 0518 AM.