Anonymous Caller What measured of 5.7x28mm? (Subject repeated due to truncation:) What is being measured of a ballistic round designated as "5.7x28mm"? I apologize if someone asked this already, but can someone explain to me what exactly are the dimensions referred to in standardized small arms ballistic designs? For instance, the M16A2 uses 5.56x45mm NATO rounds. What do those numbers refer to exactly? A better example would be the thing I'm actually looking up: FN Herstal produces these 5.7x28mm cartridges. According to someone's forum post, the 5.7mm is the shell casing width, and 28mm is the shell casing length. However, looking at the images on this page (http://www.fnmfg.com/lawenf/ss190/ss190.htm) as well as considering the cartridge length of 1.59in/40.5mm, the dimensions would suggest the cartridge (shell+bullet) looks like a toothpick, which is quite unlike what we see in the images on that page. This makes me suspicious of the accuracy of the forum post information. Also, 5.7mm seems to be approximately .224 caliber. I've recently fired a .22 handgun and have noted that the cartridges/shell casings are quite tiny. I wouldn't expect that to be used in a SMG (or mini-assault rifle) or a military/law-enforcement grade handgun such as the Five-seveN. Instead, I would expect minimum caliber to be at least 9mm, or aboout .354. The 5.56mm NATO rounds are even smaller (thinner). So do the M16A2s fire "peas"?--although quite elongated--okay, needles? Something doesn't make sense here. I've spent hours searching on Google last night, but no one explains with authority what is actually being measured by the name. If this is answered, please don't forget to describe the second number as well. Thanks! :3 Sagara Sousuke The dimensions you are referring to are for the bullet width x length, not the casing. So, the bullet you describe is 5.7 mm in diameter and 28 mm long. The casing is typically longer and wider than that. As you can see, the casing actually tapers into a neck into which the bullet is pressed. The website you mention has a good picture of the bullet itself towards the middle, and at the top, shows the bullets mated to their casings. While you have fired a .22 caliber (probably Long Rifle) round and were unimpressed, imagine if you were significantly increase the explosive power behind it. As you can see, the casing holding the powder is longer than a .22LR and significantly wider, so the powder charge, and hence, muzzle velocity is much greater. In this case, 715 m/s, or roughly 2.1 times the speed of sound. Also, the bullet, in this case is considerably longer, while for a 22, the bullet is not much longer than it is wide. With increased volume, comes increased mass. From there, it is a simple matter of math: Kinetic Energy = 1/2 * Mass * Velocity^2 Increased mass and greatly increased velocity results in significantly improved kinetic energy, which is then used to penetrate and perforate your target. While a 9 mm handgun round has a larger bullet at roughly 8 grams (versus the 5.7x28mm's 6 grams), the bullet has less powder behind it, and therefore goes considerably slower. Your typical 9 mm pistol fires at 1200 fps (366 m/s, or barely sonic at Mach 1.06). As the equation shows, if you double the mass of the bullet, you only double the energy, but if you can double its velocity, you increase its energy by a factor of 4. The other important factor is that small bullets travelling quickly are more accurate than large bullets travelling slowly. It seems that Fabrique Nationale (FN) has developed the 5.7x28mm round specifically for their line of sub-machine guns, which typically fire handgun rounds like the 9 mm Uzi and HK MP5, or .45 ACP MAC 10, in order to bridge the gap between handgun ammunition and rifle ammunition such as the .223 caliber (5.56x45 mm NATO) rounds fired by the M-16 (muzzle velocities up to 975 m/s or faster than Mach 2.8).