Points to note:
If the Super X shells are the discontinued Compression Formed hulls the working pressures will most likely be at the high upper end compared to other US and Euro hulls.
What is dram equivalent of these shells and the payload?
What is the weight of the gun you are going to fire them out of? And if a vintage gun, perhaps you should measure chamber cone diameter and length.
Back in the day, after plastic cases were introduced by Eley - the factory changed many of their product range of traditional 2 1/2" shells to 2 3/4" hulls but designated as suitable for 2 1/2 " chambered guns. This was presumably to facilitate their producing crimp closure shells. Doubtless these were developed and tested to achieve the appropriate pressures for the shorter chambers.
(I recall seeing some pretty poor quality Eley factory loads in 16 gauge - including cork overshot wads underneath the crimp closure.) There were a few instances of guns with short cones or very sharp step to bore "shooting loose" ie needed re-jointing.
The barrels of your gun is likely to withstand the Super X pressures but if you intend to shoot many of them out of a lightweight s x s 20G you better expect to be asking the readership who would they recommend to rejoint the gun or do stock repairs.
