A good portion of the reason for less trimming is that the case usually shortens up when fireformed.
The contention is that the sharper shoulder unhibits the forward flow of brass. I have not had the problems many do with stretching cases (except in Lee Enfields). Until I actually run a comparison of my own, I'm not prepared to say it is or is not effective. I only know I can shoot my 303 target rifle (built on a P14)at least 8 times with one piece of brass and no trimming is required. I had thought of rechambering it to the 303 Epps and doing a comparison but it shoots too well as it is to mess with it.
Often, the owners of AI rifles are better at setting their sizers up and this will account for some improvement as well. GD
