Since I already typed up a long answer in another FN five seven thread I will simply copy and paste it here:
What I don't understand is why the military doesn't just use armor defeating 9mm rounds. Make it a steel core and make the ogive more pointy and it could work.
Because the smaller a bullet diameter the less energy is needed for penetration of thin barriers. So a .22 zipping along with a hard core has quite a bit of penetration for the low amount of energy. Since the case is also of a small diameter the capacity potential is very high. Since the energy requirements are low for the level of penetration there is also little recoil.
That is essentialy
what the weapon platform the round was designed for is, so it comes as no surprise. The P90 is low recoiling high rate of fire weapon that shoots weak rounds that can still penetrate some body armor.
To try to compensate for the horrible terminal performance of a .22 the bullet is designed to bend and then tumble end over end once it gets through the body armor, giving a greater than .22 caliber wound channel.
That sometimes works and sometimes does not, and the perfomance of the round is highly dependent on it.
Since the 'civilian' (police are civilians in America, I guess they forgot) ammo is not supposed to work that way, it is little more than a .22 magnum unless different ammo is used.
So the platform is mainly for support personel in a warzone. They can 'spray' a high volume of weak rounds at soldiers in body armor from close range using a compact platform suitable in confined spaces like vehicles etc
However many have decided to just use M4s for that role rather than a specialized weapon that has different ammo requirements. The M4 has higher recoil and a lower rate of fire, but more devestating rounds, and is already widely fielded by troops using a widely fielded NATO caliber. So why would they keep the 5.7x28 around?
The 5.7 pistol was just designed to use the same ammo as the p90, and be a companion to the P90 as a main weapon without seperate ammo requirements. The P90 as a main weapon is unnecessary since something like an M4 is effective, cheaper, and already widely available, a companion pistol using the same ammo is also not helpful.
Personaly I think the 5.7x28mm is a very specialized round intended to be used from a platform with a high rate of fire. Since full auto is not civilian legal (modern platforms) it can never live up to its design in the civilian market, nevermind the seperate ammo.
I think the only thing the round has going for it is low recoil and high penetration, enabling very high rates of accurate full auto fire that still puts something through soft armor.
Since civilians do not use full auto fire, there is much more effective rounds for both that task and many others.
Another thing potentialy going for it is the interesting bullet design, but those are generaly not sold to the public.
Even if they were, and you had a use for them,
a semi auto in that role firing such a weak caliber is just asking to take incoming rounds before a threat is stopped. They may penetrate soft armor with the correct ammo, but the terminal performance per round is low so full auto is required to reliably stop a threat from continuing to return fire.
So if new full auto firearms become legal, a full auto 5.7 would fill a niche. Otherwise its just an expensive rodent gun in my opinion.
With semi auto fire every round needs to be more effective. There is more devastating AP pistol rounds than the 5.7 if they were necessary (of course banned by federal law.)
Of course there will be cases of it working. If many people felt confident in a .22LR and trained hard with them and they were widely fielded, reports of them working many times would also exist. That would not make it an ideal defensive round, just a widely used round by well trained individuals.
I still stand by that post.
If it was legal to convert the pistol to full auto, and the rounds the caliber was designed to utilize were widely available and legal, I could see great potential with the low recoiling round. You could keep a lot of firepower on target. Otherwise I will take more effective calibers with a bit more recoil for semi-auto fire. You give up too much without gaining enough to compensate using civilian rounds and semi auto fire with the 5.7x28mm cartridge.
The whole purpose of the round is controllability on full auto fire using rounds that can penetrate soft armor, and then rapidly tumble to compensate for low energy and diameter. So civilians give up the best attributes of the round, and retain the poor ones. You end up with just a fancy .22 magnum.
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