what I have noticed, I had two AR's on the range a spikes carbine and my adams arms mid-length doing mag for mag switching.

the impluse to the rear did not change, neither really had much.

it was in the jump of the rifle.
the spikes seemed to want to jump sidewise and a bit, I suspect this is because the bolt is moving under more pressure.
my AA mid length I don't feel that jump, and My follow up shots come much faster..