I agree that the differences should be relatively small - say as found by Bell. I imagine that the hull mouth opening into the cone does add a small amount of resistance to wad movement, just enough to bump peak pressure a few %. Burning pressure is a big factor in nitro powder burn rate. As a consequence, nitro powder gets increasingly touchy as pressure goes up. Pressure too low and it doesn't burn well. Pressure too high and it becomes intolerant of small variations in burn factors.