Problem might be that the current 5.56 rounds are not soft enough... the bullets aren't tumbling when they hit the target and not breaking apart when they're in the target.
The main problem from what I've read is that they sail right through their targets.

So the problem is with the ammo, not the rifle.

It never made sense to me why they would use armor-piercing rounds on insurgents who, for the most part, aren't wearing body armor. I know that 77gr. Mk 262 Mod 1 match ammo is supposed to fragment reliably every time, but I don't think everyone gets it. Do they?

Personally that's what I think it is. The ammo. It's not necessarily the caliber.
5.56 wounds are supposed to be REAL bad if the bullet in fact tumbles and breaks. You have yourself a small entry wound and a grape fruit sized exit wound. That's what we were taught. From what I saw from my shooting (I could be wrong...) at least up to ranges out to 300m, the green tips seemed to have a flatter trajectory. May mean the bullet is more stable... and maybe it's too stable.

I'm an engineer, so I let raw data do the talking. Take a look at the attached matrix of performance of the current 5.56x45mm ammo (newer M193 vs older M855) used by the US military today. I highlighted the 300m range data because it reflects our desired expectation to be able to engage targets effectively at the furthest possible distance. At 300m, the muzzle velocity is reduced by 37% for the new M193, and 32% for the older M855. Muzzle velocity is directly proportional to energy (i.e. stopping power) of a bullet. At 300m, both rounds have from 475-500 ft-lbs of energy behind them.

To visualize this, imagine driving a nail through silly puddy with a sledge hammer (assuming the hammer doesn't touch the puddy).

The point I'm trying to make is that at these ranges, this bullet will not tumble on its own (which is an aerodynamics problem I won't bore you with), because they retain most of their energy at the desired range (and in turn, their spin, which keeps it flying straight).

Also, it is vastly more economical to supply a lot of what you already have than to convert to an expensive new round AND convert your rifles to said round. We could easily avoid the conversion and increase lethality by using jacketed hollow point (JHP) rounds, but they are illegal to use in war.

Wouldn't JHPs also have less chance of penetrating body armor if you come up against it?

Originally Posted by mmarsh

The question is will they? I have lots of complaints about the lack of "stopping power" of 5.56 munitions -Personally I wouldn't want to be hit by one thats IMHO.

I am not saying I would wanna get hit with one ethier I would not wanna get hit with any round. I am just saying it may not be the ideal round for man stopping.

Originally Posted by major liability

Wouldn't JHPs also have less chance of penetrating body armor if you come up against it?

Yes, however, some rifle bullet designs are such that they can achieve the "mushroom" effect without the need of a true hollow point, circumventing the 1899 Hague Convention limitations on the use of expanding munitions.

Again, the need for a softer bullet.

The current 5.56 ammo, both the regular ones and the green tipped ones overpenetrate the human body. they dont cause enough internal damage. on the other hand they are very very accurate and pnetrate body armor preetey well... If you have a less stable ball than yes it would thumble inside the target and and cause a big mess, but the chanses of hitting at long ranges get slimmer. Its a known problem ever since the m16 is used. saying you wouldent want t get hit by one is hardly a good point. I wouldent want to get hit by a laundry machine(youd be surprised how much of a real possibilety that is) but that dosent make laundry machines an effective rifle round.

As long as it's stable enough to hit targets at 250m, I don't think it'll be much of an issue with the regular infantryman.