5.7x28mm Ballistics Chart
The 5.7x28mm caliber was developed after NATO requested ammunition that had armor piercing and longer range capabilities than 9mm cartridges. The first manufacturers of 5.7x28mm was FN who designed it for use in their FN57 pistol and P90 Submachine guns. Ruger quickly made their own Ruger57 as well. The reason why NATO has not fully adopted the round is because Heckler & Koch lost the contract deal to develop their 4.6x30mm cartridge. H&K being from Germany (a key member of NATO) the entire country rejected the standardization.
40 countries today still used the 5.7x28mm as standard ammunition due to its unique low recoil and long range capabilities. Its has since then went into the civilian market where it has become increasingly popular.
What makes the 5.7x28mm so unique is how incredibly light of a round it is. It's only 27 grains which gives it a greater muzzle velocity, allowing it to be a faster and flatter flying round. The high muzzle velocity is what gives it its armor piercing capabilities. Being a flatter flying round, it is also a lot more accurate than 9mm at longer distances. Check out the trajectory chart below.
Types of 5.7x28mm
Ballistic Gel Test
5.7x28mm vs 5.56x45
Guns that shoot 5.7x28mm
PSA 5.7 Rock