It is always said that the improvement of AI is greater case life and less bolt thrust. Besides case forming, are there any downsides? Why have the mainstream manufacturers not adopted the 40� shoulder for new calibers?

Turn of the last century cartridges had lots of taper and low shoulder angles, it seems to me. WSM & RUM have 25� I think. Why is 40� not more beneficial in their thinking or experience?

Anybody know?

The feeling is that the 40 deg shoulder doesn't feed as reliably as a more sloped shoulders. I've used it on wildcats from .243 to .375 without problems.

Sharp shouldered cases are more difficult to make factory cases for.This probably factors into the manufactures not adopting "improved" calibres

Dave,

The other gentlemen have covered most of the points why factories do not use 40 degree shoulders.

The primary thing that they must consider is whether they can make brass without too high of a scrap rate. For that reason they typically like to stop at 30 degree shoulders. There have been a couple of 35 degree shoulders.

Also, they do not find it easy to form cases with the small amount of body taper that Ackley cases have.

I hope this helps you understand.

Good Hunting !