It's my understanding that the chamber pressure is higher because they use a faster burning powder. There are several reasons for this, one is to get as much velocity as possible out of a short barrel M4. They also worry about muzzle flash, any unburnt powder will create a much larger flash. The muzzle flash not only gives away your position to the enemy, but it also blinds the shooter when making a nightime assault while wearing night vision goggles.