Bottom line in the debate is powder.
The M16 and M4 have different barrel lengths.
If the powder gets a bullet up to speed in the M4 short barrel it will cause overpressure in the longer M16 barrel. It it gets the round up to speed in the longer barrel it will be under speed in the shorter M4 barrel.
Speed - spin - accuracy. Can’t get the same performance from both rifles with the same round.
I chronographed .223American eagle 55 grain fmj at 2828 fps out of a 16 inch barrel. I thought I would get 2900 fps.
The American Eagle box stated 3240 fps.
You got a valid point.
"The M16 and M4 have different barrel lengths." Yes
"If the powder gets a bullet up to speed in the M4 short barrel it will cause overpressure in the longer M16 barrel." Not true.
The chamber pressure is measured in a test barrel. By the time the bullets traverse the length of the shorter M4 barrels, they are already on the down slope side of the pressure curve and the pressure will continue to decrease no matter how much longer the barrel may be. Velocity may increase, even though pressure is decreasing.
"If it gets the round up to speed in the longer barrel it will be under speed in the shorter M4 barrel." (WTF?) Bullet motion is measured by rotation and velocity, not speed. 5.56mm ammunition average velocity and maximum chamber pressure is specified and determined in a 20 inch length test barrel, not the barrel length of individual weapons.
"Speed - spin - accuracy. Can’t get the same performance from both rifles with the same round." Please repeat the obvious over and over until you have it memorized.
That is the best answer I’ve seen yet.