The FN 5.7 is an interesting concept which basis a lot of sales on it's potential with the right bullets to go through level IIIA soft body armour. 5.7mm is .22 centrefire in rifle ammo and their pistol ammo more or less comes in at a low centrefire .22 PPC velocities. Legally sold ammo is of the type using bullets intended for varmint or target paper shooting. They break up easily upon penetration. However for true kevlar perfomance you would need to re-load the bullets by pulling the original bullets or simply buy the rounds in empty case format and re-load with suitable armour piercing rounds which cannot be legally bought.
Therefore for "sales" purposes and media reporting it has a cache of effectiveness it does'nt really have ... particularly allowing that the Muslim US Army Pschiatrist who went nuts with one on Ft Hood ... he bought one because he was sold the idea that this pistol was kevlar defeating when infact the factory ammo sold to him was not ... and because this calibre only works well if the bullet either becomes "frangible" and breaks up upon penetration or if it tumbles during penetration ... because for wound channel creation it is only .22 in diameter.
What it does have going for it though is that being so small in diameter it is better placed for penetrating carbon fibres blended into kevlar ... which is a substance that primarily works because of the strength of the fibre utilised into a tight weave and it can shoot through these at high velocities.
However to understand whether or not larger calibre pistols are therefore "not as good" at this task you need to consider what else is out there.
Here is a good link to an article about a specialist distribution company Le Mas Ltd who sell specialist ammo created by another company specifically for anti kevlar use.
http://www.defensereview.com/le-mas...ogy-ammo-vs-ads-transparent-armor-afji-video/
Forget about the "Blended Metals" name for the bullet makers. They are not producing hybrid metal bullets to do the job. What they are doing is creating rounds that are a lot lighter bulets at higher speed for standard pistol calibres which then share the same properties of the 5.7mm ammo and applying special coatings to the bullet.
For the 10mm the Le Mas L.E.A./Military ammo is a 77g bullet doing 2500 fps and a 33g bullet doing 3000 fps. These are small bullets for a 10mm but because they are so light you can shoot them safely in the pistol brass because pressure is still controlled. I would however have recoil springs on the stiffer side for use with hot 180 JHP ammo if wanting to interchange to Le Mas. Le Mas follow the same strategy with 9mm and with .40 S&W and .45 ACP.
The main thing when you have such "light" bullets in large diameter calibres is that the sectional density of the round ( basically how long they are or how much "core" there is ) is very low. As a result whilst the bullets can be used to penetrate the kevlar once they hit flesh the lack of sectional density induces the bullet to break up into pieces ... thereby controlling over penetration and hopefully creating a bigger wound channel.
The trick is though with these type of bullets for this type of application is to have the bullet coated properly to retain a slippy shape for penetrating kevlar fibres and to keep the metal used in the bullet from "mushrooming" due to the speed of the bullet and the coefficient of friction of air before it hits the intended target.
Their secret therefore is not so much the blending of metals but rather the application of coatings. Teflon is the most common form of a heat shield coating and moly being applied on top to enable the bullets to ride down the barrel slickly and to counter act the effects of the teflon which would not perform like copper brass jackets and melt and mold as per normal to the barrels rifling.
So the bottom line is that there are many pistols out there which can do a better job at sorting kevlar than the FN 5.7 and had the FT Hood Pscycho been using a 9mm/40/45 with Special Project Ammo there would likely have been far more deaths as the wound channels would have been deeper ... and in point of fact the personnel on base were not wearing kevlar ... as the C.O. said ... they were not armed or wearing armour because this is our "home".
Compare the flat shooting fast 10mm which also has a great kevlar option which does'nt need to rely on "tumbling" with the 5.7 and you can see why hardly anyone who has to be at the sharp end of a gunfight in the military or LEA is choosing them. They might have them. We do. They are'nt choosing them though.
Personally I think the "system" the FBI had with the 10mm and the H&K MP5/10 is the best I have used/encountered for a single calibre "long and short" combination ... it is just a shame that the P.C. considerations of today meant it was taken out of service.
Lol ... I would love to have known what Jeff Cooper would have said about the FN 5.7 mm system ( because it was combined with a "short and long" option for a NATO weapon trial ). Jeff Cooper created the 10mm auto cartridge to give greater capacity and as good performance or better than a .45 ACP. After John Moses Browning he is one of the few people who (I think) knew what they were doing in calibre-weapon design.
Eugine Stoner and the 5.56mm "Revolution" has probably caused more "failure to kill quickly" loses on our side than can ever be counted or known ... no wonder 7.62 battle rifles are being comissioned quickly for S'tan.
It will soon be 30 years to the day I went to the Falklands ... no one came out of that war saying that their FN's were not up to the task. The SAS/SBS who had 5.56 were the first to appreciate how good the Argentine FN FAL's with folding stocks were ... a lot of those came back home with us with the guys who had picked them up.
Lord knows how bad it will get if we went down the 5.7mm route ...