As far as the 5.7x28 is concerned....two things contribute to its lack of popularity: the gun is expensive and up until now there was only a single pistol chambered for it, FN’s FiveSeven. Ammo is expensive relative to more common pistol ammo like the 9mm and not available everywhere. It is also seen by many as simply a glorified .22 magnum though it is far more than that. Perhaps this will change with the advent of Ruger’s 57 pistol. Less expensive, maybe it will stimulate other ammo companies to start making the 5.7x28
To be on the wire is life. The rest is waiting. Karl Wallenda
