I am with you on the .303--and that there are other causes of case stretch.
In fact, there are two common causes of case stretch even without considering the design of the rifle itself. One is the lengthwise stretching caused by sizing. The other is actual forward flow of the brass.
The first is exacerbated by a mismatch in sizing die and chamber. Unless cut by very carefully made custom reamers, both vary somewhat. If you combine a relatively small sizing die and a large chamber, brass will stretch noticeably with each sizing. This is purely a matter of forcing the larger case into a smaller space: The brass has to go somewhere. I have experienced this with several rifles, most notably a .270 that I eventually had rebarreled to 6.5x55 partly because of that problem.
But some cases do have a tendency to stretch more than others, due to brass flow. Those with longer, sloping shoulders tend to do so. And it is indeed a matter of brass flow. This is proven by the thickening of necks, especially at the base. This is normaly diagnosed by sticking a bullet in the necks of fired cases. If the bullet doesn't want to slide in, the necks are thickening somewhere, and it WILL cause problems.
Whatever the reason, I have indeed seen much less need to trim cases in the "improved" rifles I've owned, as well as some factory rounds with sharper shoulders.
