Seem to recall at some point in time this "Targo" game was promoted as "Mo-Skeet-O". Once had a Mossberg rifle model 42MC which was one of the variations of the basic model this 42TR was based on. I recall Remington making rifles for this "Sport" with the forward portion of the bbl enlarged to about a 5/16" bore & choked, known as the "Routledge (SP?) bore.
In Burrard he gives standard British loads for the .410 as;
3" = 9/16oz
2½" = 3/8oz
2" = 5/16oz
These would have been for the paper case. It is well established that an all brass case of a given length has more internal capacity than a paper one, thus can receive a heavier load. This would of course most often give a load which exceeded the load for which the gun was proofed. Burrard did list loads under the "Low Velocity" principal in which the powder charge was appropriately reduced to not exceed proof parameters in which the 3" load was increased to 11/16oz, the 2½" to ½oz & the 2" to 3/8oz. In a country such as England with proof laws then I believe that a full 2½" load put up in a 2" brass case would only be approved for use in the 2½" chamber.
Of course for the US (us) the FF factor applies ie If it "Fits-Fire" it.
