There’s a lot going on when gas hits the carrier in the M16/AR15.
Before the gas starts it’s way down the port, the case head has slammed the bolt into the locking lugs with somewhere around 55-60,000 psi and is holding it there with what ever the chamber/bore pressure is, all the way to the muzzle.
Once the gas gets to the chamber in the carrier, there is a fight between the carrier and the bolt over who is going to move first and farthest. The bolt is supported by not only the pressure in the barrel, but also the brass. For the bolt to move forward, it needs to overcome that gas pressure, but also crush the brass after overcoming the friction of the case wall against the chamber.
The carrier pushed by the same pressure as the bolt, needs to not only overcome the pressures on the bolt face to twist the case in the chamber to move back and begin the cycling sequence, where the carrier also needs to overcome the buffer weight and spring tension.
The actual cycling of the carrier is accomplished by a very short burst of air pressure and the inertia of that blast. As soon as the carrier travels enough for the tube to come out of the key, all pressure on the carrier stops.
The bolt is held in place by the expanded brass gripping the chamber walls created by the pressure in the bore until the bullet exits the muzzle, allowing the pressure to drop. Then the carrier can jerk the spent case out.
Bolt speed and the time it takes to move far enough to unlock the bolt, vs how long it takes for the bullet to exit the muzzle is what tuning is all about. The biggest factor is gas pressure and volume. The most important control of that speed is how fast the gas is applied. This is controlled by the size of the port, distance to the carrier, and pressure. Keep in mind the gas on the way to the port is still expanding, so powder burn rate can be a factor.
Bottom line. There is a lot going on with pieces parts banging around and moving back and forth while the bullet is traveling down the barrel. Kind of like some one hitting the action on a bolt action 3-4 time with a hammer between the time you pull the trigger and the bullet exits the muzzle.
The cycling of an AR is pretty violent.
One thing else to consider. Shooting sub-sonic can change your thinking on tuning. Adding a suppressor creates more variables. Most people will agree that adding a suppressor will give a little boost in velocity. But consider this concerning an AR. The boost in velocity comes from pressure inside the suppressor after the bullet leaves the muzzle. It’s not that it adds to the velocity, it just lessens the velocity drop after leaving the muzzle. Without the suppressor, the bullet leaves the muzzle and pressure behind the muzzle drops to near zero almost immediately. With the suppressor there is a tail wind longer. The bullet decelerates less In the first foot of travel to the chronograph.
What happens when the action opens before the bullet leaves the Suppressor?
You lose the “boost” of the suppressor. All the back pressure creating the tail wind, blows out the ejection port. Simply changing spring weight, or anything else that will delay the bolt opening the chamber will affect velocity. It just doesn’t fall outside of acceptable variations until you slow things down.