OFB manufactured standard 5.56 x 45 is already underpowered, when you take 5.56 x 45 (energy 1125 ft/lb )and cut it down to 30mm case length, your energy drops to something around 750 ft/lb. 5.56 nato's main advantage has been it's high velocity which compensates for it's small diameter, but when you cut down the case length, you end up with crappy terminal ballistics and crappy velocity. A 9mm round still will travel at a lower velocity than the 5.56 x 30 but will have a projectile almost twice the diameter, the bullet weight at 9.54 grains for ofb 9mm fmj is twice of the 5.56, hence it will be much more effective at close range. Your optics wont do anything if your bullet cannot take down the enemy, unless you plan of flinging optics to take down your enemy...