my $.02 on the whole 5.7mm thing.
remember, all this started in the late 1980s as the military Personal Defense Weapon project.
the idea was to create light, compact and handy weapon, suitable for non-riflemen use, and capable to hurt enemies wearing body protection means.
The 5.7x28mm ammo was devised as a compromise between low recoil, good penetration and acceptable stopping power. but remember, it was acceptable for military - at least for those who do the thinking for military.
The police / SWAT operations are completely different strory, where stopping power is a major factor, and you can trade the range and some weigh for better stopping power, because you don't have to haul your PDW for weeks, while loading 5" shells into your mortar or driving helo at treetops.
as for design, P90 is a siple bolowback design with interesting magazine design (with somewhat questionable reliability) and 100% ambidextrous handling.
The Five-seveN is a delayed blowback pistol.
Both, IMO, had very little to do in police work, unless you want to look like Stargate tooper.
if you need good penetration - just buy some 7N21 or 7N31 9x19mm AP ammo for your loved H&K MP5 or Sig P226 or Glock17. This had stopping power of standard 9mm bal ammo (not that much, but still better than 5.7mm) and penetration at least equivalent to 5.7mm. And you still have weapon compatible with best available JHP slugs for unarmored adversaries. The maximum range is slightly lower than of 5.7mm, especially from P90, but why you need 200m range in SWAT operations? if it came to that sort of things, you'd better go for full-size rifle in .223 or .308, that will do much better...
so, again, IMO the whole 5.7mm thing belongs to the military PDW world, and is far from being good for police / swat job.
