Back in the early 1960's I shot a 20 gauge Flues quite a bit. Chambers had been lengthened to 2 3/4" & not knowing any better at the time I shot it mostly with off the shelf 2DE-1oz shot loads. It took them all in stride. "IF" I were using it today I would limit it to the 2DE-7/8oz loads.
This was a fairly late Flues with a 6lb weight. Some earlier Flues' are around 5lb & may have thinner side walls in the frame which is where they crack.
Keep in mind "Pressure" bursts barrels, "Recoil" cracks frames & stocks. For the Flues you do not want Heavy nor Fast shot loads, its not really so much about the pressure.
