A similar tale.
Before I start I should state that my ancient A.Gale 10g pictured on p11 of this thread has been back to Birmingham for reproof and passed 1200 bar proof.
Homeloaders among us will remember the shortage of Alliant Steel powder a few years ago. I was getting low on powder for my own loads and so bought a couple of boxes of Remington Sportsman 39g (1 3/8 oz) No.2 . The barrel flats are stamped 1 1/2oz and that's what I've always fed the old girl so figured 1 3/8 oz would be fine.
First thing I noticed about the Remingtons was the ferocious recoil. I returned from the marsh with a bruised cheek, 3 duck for 13 shots and none of them were killed clean. Second time out with the new shells resulted in lots of misses and 4 duck, 1 clean kill, 2 runners and an unusual one that flew on 200yds and then dropped dead. After plucking I noticed that each bird only had one or two hits and that some of the pellets had passed right through the bird.
As JJs Dad did, I patterned a few shots from each barrel (1/4 and 1/2 choke) and was getting open patterns averaging 39% (67 pellets) in a 30" circle at 40yds. The patterns were so open I couldn't tell where the centre was but they looked to be shooting a bit low on my un-scientific sheets of wall paper.
I was sending some homeloads off to the proof house for testing and so included some of the Remingtons. The Remington box said they did 1500fps but I've never believed manufacturers claimed velocities. However, when the results came back they were doing over 1500fps and failed the CIP velocity upper limit. (Test sheet attached.)
I can only conclude that the high velocity was blowing the pattern apart. Happily, Alliant powders were soon available again, I was able to revert to my nice slow 1 1/2 oz No 3.