The 5.7x28 shines only because of the FN P90. Anyone who is familiar with this caliber knows its attributes, especially when compared with traditional SMG's and the 9mm or .40 caliber rounds which they fire. It approximates rifle caliber penetration in a SMG, but it can't compare to 5.56x45. Its strength is that the weapon that fires it is much smaller/more compact than traditional carbines. Besides the 16inch PS90, as its bullpup configuration still make it extremely compact, there is nothing this caliber has to offer in traditional carbine like the M1 or AR15. I guess if you have a P90/ FN5.7 it would be nice to be able to have ammo commanlitly, but from a practical/lethality stand point its ridiculous. That being said, FN 5.7 is as close as your are going to get to a M1 chambered in Johnson Spitfire....take a handful of the Johnson and FN rounds and mix em up and try to sort them out....very similar. Besides the PS90, a carbine which is the same size as an AR15 chambered for FN 5.7 as opposed to 5.56 is ridiculous.