Hodgdon publishes some Longshot loads with Fiocchi hulls, but unfortunately with bismuth shot. I know you can't interchange load data, but you can get clues. They are at 26 grains for 8,800 psi with 1oz bismuth.
But it could simply be that you are on the edge of unstable burn territory and a less than ideal crimp isn't letting the burn ramp up before the burn volume starts really expanding?
Personally I always wondered whether Longshot was touchy just because I never see lower velocity loads with it in official published data - usually 1,350 +. I might try a different powder.
