Just a guess here on the "higher pressure in a short hull": I'm wondering if what they mean is this:
Pressures generated in a lot of 2 3/4" reloads are not suitable for guns with short chambers. Therefore, if you use the same formula in a short hull, that still leaves you with too much pressure.
Or maybe not.
But there are BUNCHES of low pressure 12ga loads out there for 7/8 oz shot which use 2 3/4" hulls and require no filler. I reload several different recipes for my short 12's--low pressure, 7/8 oz--and I don't use filler in any of them. That's one of the great things about reloading 2 3/4" 12's for 2 1/2" guns.
