Sorry, he’s right.
Out of the FiveSeveN, the 5.7x28 is only moving at 1650 fps and has only 242 ft-lbs of energy.
The AR-15 firing the ‘weak’ 5.56x45 rifle cartridge makes 3200 fps and develops 1200ft-lbs of energy.
The heavier M1 or FAL firing the .308 Winchester or 7.62 NATO cranks out 2754 ft/sec. The reason that round is sometimes known as “the meat axe” becomes clear when you discover that it has **2422** ft-lbs of energy.
In fact, the 5.7mm isn’t that far off from 9mm velocities. The 9mm 115gr pistol bullet travels at 1300fps and has 420 ft-lbs of energy.
Sorry, you’re wrong. It’s just not that good a round.
Small, fast cartridges like these 5.7 MMJs (necked down .30 carbine) are not really new; handloaders have been enjoying them since the 50's. The little 40 grain bullets have limited penetration, but explosive expansion characteristics at moderate range. This is the first instance I've heard of the FN round used in a crime.
The empirical evidence at Ft Hood would belie that assumption.