Interesting historical information regarding the British 2" shell here. The "advantage" appears to have been for the unfortunates who did not have someone to carry their cartridges for them on long day's hunt wink
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA279#v=onepage&q&f=false
There appears to have been difficulty using Black Powder or Bulk Smokeless, and with combustion using Ballistite Dense, so there was a trial of 1 1/8 oz. 3 Dr. Eq. 2 1/2" shells in 2" chambers with long forcing cones, with little increase in pressure.

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