Great point by alloutdoors. Shotgun patterns vary so much from shot to shot that I would not worry about the possible difference made from removing the barrel. You would never be able to prove any difference if there is one. No two shotgun patterns are alike, sure the #s may be similar but the placement of the pellets within a 10, 15, 20 or 30 inch circle can and will vary shot to shot. And that is when everything else is equal which is rare under field conditions.
Now if something is different when you hunt, than it was the day you patterned, such as temperature,wind (speed or direction), altitude, humidity, or air pressure then it will affect your patterns also.
I wouldn't worry about it.