5.7x28 is often derisively referred to as ".223 short," and the comparison isn't all that far off - both use similar 0.224" diameter projectiles.
The more-effective SS190 round is curiously not for sale to civilians because it is considered "armor piercing" (funny that ordinary large-bore revolver rounds capable of doing a number on body armor are not restricted).
5.7x28 would be more affordable if the dies were readily available and you could load your own for far less than the $0.50/rd that FN sells the ammo for.
The more-effective SS190 round is curiously not for sale to civilians because it is considered "armor piercing" (funny that ordinary large-bore revolver rounds capable of doing a number on body armor are not restricted).
5.7x28 would be more affordable if the dies were readily available and you could load your own for far less than the $0.50/rd that FN sells the ammo for.