That way the gas is at a lower pressure when it is tapped off of the barrel as it is further from the chamber.
I'm not sure this is correct . They way I understand it is , it's all about the dwell time . It does not matter as much how far the gas block is away from the chamber as how much barrel there is after the gas block . Think of it like this .The bullet is plugging the barrel as it goes down the bore and as long as the bullet is in the barrel there is a lot of pressure in there . As the bullet passes the gas block the gas and pressure gets redirected down the gas tube . A carbine length gas system is 7-1/2" leaving 8-1/2" of dwell time in a 16" barrel. If you were to cut that barrel down to just in front of that gas block . The pressure would drop off immediately as the bullet left the barrel and there would not be enough pressure to operate the bolt . The opposite is true as well . If you had a 7-1/2" gas system on a 20" barrel there would be to much pressure , for to long of a time as the bullet plugged the barrel for that 12-1/2 " past the gas block . I believe the original M16 had a 20" barrel with a 12" gas system leaving that 8" of dwell time . Now there are all kinds of ways to play with that system from spring stiffness to buffer weights and the size of the gas port but generally speaking it's all about the dwell time .