Perhaps it was the thought of a 100yd biscuit tin lid. that's a huge tin of biscuits!
Never tried 100 yards but I would have thought that there were so many variables - particularly with a springer, and including the way you hold the rifle, slight cant etc, that (for a decent pellet like the JSB Exacts) any tiny variations in pellet weight would have an insignificant effect on the end result.
Perhaps the question might be "what would you be happy with" at that distance? I don't know, but for me an inch spread at 50m off a bag on a calm day with a normal PCP and pellets straight out of the tin would be good, based on what I get with my FAC Rapid. That would probably equate to over two inches at 100 yards. I would be very satisfied with what you've achieved with the HW, but then I'm easily pleased