They should have just gone back to a twenty inch barrel on the 5.56 and upped the pressure a bit. It never was a good idea to take a cartridge so dependent on velocity as the 5.56 and start turning out rifles with 16 and 14 inc barrels.
Sure, the new round gets high velocity out of a short barrel, but then you add six inches of suppressor and you are right back where you started.
I have always thought that was a big mistake. The AR-15 was built for the SCHV project. I guess they didn't bother to ask what that was later.
