A 55grn .22@2800+ has proven to be marginal in the sandbox. I sure wouldn't want to depend on a 31grn .22 going much slower.
Thats another argument brought against the 5.7, but it isn't legitimate either.
#1. You said "in the sandbox". Does the same marginal performance apply to the 5.56 round when used in a close quarters role? Close quarters is the intended use of the 5.7 weapons. The 5.56 performance issues you present are at extended range.
#2. Here is where we stop talking bullet weight and velocity. What ultimately matters is the wound, not the bullet characteristics. The 5.7 creates a larger permanent cavity than the 5.56 because the projectile has a longer length, leading to a larger wound during tumble. The 5.7 also tumbles earlier and stays in tumble longer than the 5.56mm. Take note I am NOT saying the 5.7 is superior or equal to 5.56mm. The 5.56 has the benefit of a much larger temporary cavity, whereas the TC of the 5.7 isn't large enough to aid its TE.
#3. A round only need penetrate a depth equal to the thickness of the target. The average thickness of a human torso is 9.4 inches. The penetration given by the 5.7 in the Five-seveN is 8-10 inches and 11 to 13 inches in the P-90. This is the most penetration you can get without overpenetration, and its also the most you will need in most scenarios. Therefore, the superior flesh penetration of the 5.56 doesn't help it in 90% of its applications and overpenetration is a concern. Reduced overpenetration was one of the design points of the 5.7 and one of the reasons why it is finding favor with so many protection units like the US Secret Service.
#4. That argument against the 5.7 is further disproved by its more-than-marginal performance in about fifteen shootings. No-one has survived a shot to the chest with a 5.7, whether wearing heavy winter clothing or high protection (IIIA) soft armor. 90% of the shootings involved one round and the others involved a couple rounds. You can argue against a round all you want, but I'm not believing you when the round is out there performing again and again.
#5. On top of this, the 5.7 weapons have been adopted by dozens of SF/CT units (including the French GIGN) after going through extensive ballistic testing. These units would not adopt a weapon for general use whose wound channel resembled a .22 Magnum. Most of these units are using the weapons for general use and many of them replaced their previously-used M4's and MP5's.
#6. The 5.7 round also has very low recoil and thus very high hit probability. Better shot placement means better wounding ability.
#7. The 5.7 weapons aren't restricted to a single shot anyway. The Five-seveN holds twenty rounds and the P-90 holds fifty.
A quote:
"As one who has actually used the P90 for tactical operations I can say that it is a devastating round despite its small size. From our initial training, I recall the fact that no one has survived being shot by the P90."
My opinion on the wounding ability of the 5.7 is that it is on par with other pistol calibers. However, the weapons themselves offer many advantages: lightweight, low recoil, almost no overpenetration, high capacity, and still have reasonable ergonomics.
-DmL