I’ve been reloading a big box of new Remington Gun Club hulls with feathered case mouths shot by a guy at our club through his O/U. I doubt that this demonstrates a problem with chamber length, but may instead reflect degradation in materials quality used in the hulls. To me, a case shot in a short chamber would not open fully. Based on my experience I would not hang my hat on a feathered case mouth being an effect of short chambers. My $.02.