I have been loading for a slightly tight necked 6 AI and 2-3% over standard 6mm loads is about it. When I hit peak pressure it comes up fast with only a half grain or so more powder resulting in stuck cases. I can slide a bullet into a fired round but it has a little more feel to it than I like. PPU cases would max out about a grain or grain and a half faster than Winchester cases. I am going to try skim turning the necks on the PPU brass as an added safety factor but it may just be the heavier brass and lesser capacity. With 100 grain bullets my comfort zone stops at around 3,350 fps. although others have reported 3,400 fps. I don't believe these were pressure tested loads. R26 gets there easily but hits a peak pressure (probably close to 70,000 psi.) at 3,400 fps which several other powders can reach too: H4831, H1000, 7828, VV 560, R22 etc.
I do believe there is such a thing as a fast barrel but more typically it is a tighter chamber. At some point I will check it out more thoroughly and maybe do a chamber cast or see if I can get the reamer specifications (borrowed from a silhouette shooter). My 1-8 twist has a short throat too which can increase pressure. I may extend the throat and open up the magazine but it shoots so well with 95-105 grain bullets I have little need to try this.
Could be my imagination but it seems to shoot flatter and get more of a reaction from deer when hit than the 243 Win does. And I am running the Winchester pretty fast at around 3,150 fps with 95 Noslers and R26.
