Well, the common sense answer is "it depends". The cartridge life issues and other considerations have been well documented and the rifle looneyism exhibited on this site particularly makes this discussion almost a constant drumbeat at times. However, as far as AI rounds being actually "improved" depends on what case you start with.
I've seen several rounds "Ackley'd" over the years that benefitted very little by the addition of sharper shoulders, etc.... simply due to the fact they already had a fairly sharp shoulder to start with and moving it forward gained very little in powder capacity. On the other hand, a good example would be my 300 H&H Improved case, which goes from a gently sloping case to basically a 300 Weatherby case when fireformed, which is a fairly substantial increase in my case. This also results in a fair number of split cases the way I do it. My 223 AI gained a pretty substantial amount in velocity also and the accuracy of the rifle is amazing at times- especially considering I am the one shooting it.
I read in a Nosler reloading book one time about fireforming cases and the author- a well known gun writer and tester who I've forgotten his name at the moment- stated that fireforming can be some of the most accurate your rifle will ever shoot. He wasn't sure why this was so, but his experience- which was substantial- was that this was often the case. I have found this to be true with my rifles. My 300 H&H Improved, 223AI, and a couple others are amazingly accurate while fireforming. Slightly more even than when I load the formed cases and shoot them after. Has anyone else had this experience?
Sheister, to answer your last paragraph. I have very limited experience with improved cartridges, but have found this to be true when fireforming for my .375AI. At 100 yards, it was just making a ragged hole. Wish I had measured that 20 round group! memtb
