It is amazing to see how worked up folks get about this round. Most everyone seems to agree that the fiveseven pistol and the PS90 are well built firearms, but debating the ballistics of the 5.7X28 round seems to bring out an ugly side in folks.
Disclaimer, I am not an expert in ballistics, or much of anything really, but I have shot the caliber a lot and have done my own testing. Everything here is pure opinion, but those of you interested in this platform, might find some of it useful.
For the 5.7X28mm to function at its intended level, it needs a lot of velocity. The bullet needs to be pushed past 2000fps to achieve the penetration, yawing, and wound channel necessary to do its job. Almost all commercially manufactured ammunition (I'm not sure about American Eagle), will meet this standard when fired from the PS90, but through the shorter barrel of the fiveseven pistol, it can be a tougher proposition.
From what I have read, the 5.7X28 caliber was designed to be a NATO replacement for 9mm and met all the technical standards but for political reasons, the caliber was never formally adopted. It was designed to be fired in full automatic mode from the PS90 and the pistol was to act as a companion to the PS90, but the round was optimized for the the PS90 as a very controllable round fired from an SBR in full auto mode. It is the FiveseveN pistol which has the most controversy, as the shorter barrel makes it tough to achieve the hyper-velocity necessary to make the platform function as intended.
FN limits sale of the SS190AP round to law enforcement. This round has a steel penetrator core. Honestly, in my humble opinion, it is not a terribly impressive round. 31 grain bullet at around 2100 FPS. It will get the job done, certainly, but is not the mythical magic bullet politicians have made it out to be. It can be purchased fairly easily, although it is quite expensive. In my opinion, it is silly that it is prohibited and I would not buy it if it was legal to carry as there are better options.
The round most of us are familiar with is the FN sporting round, SS197SR with the blue tip. I think this is the round Maj. Hassan was using but I am not certain. This is a good target round and functions great, but only pushes a 40 grain bullet at 1650 FPS or so. Not nearly fast enough. This is where folks start arguing the .22 magnum theory.
The key is to get ammo that is hot enough to create the necessary wound cavity and to get adequate penetration. I am fond of the T6B round or the SM4 round manufactured by Elite Ammunition. These rounds are pushing a 28 grain bullet at around 2600FPS from the FiveseveN pistol. Now you're in business. The difference of nearly 1000FPS between the SS197SR and the TB6 takes the 5.7x28 from a glorified .22 magnum to a serious caliber, able to hold its own with the big boys. I have shot some pork shoulders wrapped in denim to get an idea of what it might do in the real world, and while my testing is far from scientific, I was impressed enough to carry one regularly.
As I have said before, it is not for everyone, but to me it is an interesting option. Flat shooting, very accurate and minimal recoil lets me fire fast and stay on target.