Sorry, needed to be in front of my office PC for this..
Tightest cartridge i used was a Clever T3, 28g, 7 UK shot. At 15 yards it had a 10" diameter pattern.
Other examples, plas wad at 15 yards:
Fiocchi FBlu, 28g, 7uk shot = 12" pattern
Hull Pro One, 28g, 8 shot = 14" pattern
Most average 28g, 8 shot shells are around 16"
Hull ProPiston 28g, 9 shot = 20"
Express 21g fibre 7.5 shot = 21"
Hope this helps..
Apologies Dan, Pro Pistons are NEARLY as wide spreading as the Express. However, for all their hype, high cost and tiny pellets I had hoped for more value from them.