As this topic proceeds the information is being presented piecemeal and as John E rightly says it is hard to determine or pinpoint the actual site of restriction.
To start from the ground and work upwards you should have the gun chambers measured accurately for internal diameter - it is not infrequent to find the chambers of a double gun do not match up perfectly. That would be of little account if using plastic, paper or even thin walled drawn brass cases which are easily resized. This point is important when using thick walled cases which have been turned down from solid bar.!!
The discrepancy in the dimensions amongst the cases may indicate that they were not all produced as a single batch or may have been fired in different guns with the result of different points of interference when chambered in your gun. So once again check the point of restriction for each individual case in each chamber. Also check that each cartridge head rim fits within the chamber rim.
To keep floggin' the dead hoss, you could also mike the internal dimensions of the brass cases to try and identify the zone of enlargement on the outer wall.
Solid brass cases with thick walls are easier and cheaper to resize by "sand and polish" as pointed out above - and assuming that there is no weakening of the "stretched" wall - they should be safe enough for use with black powder.
The few makers of turned brass cases usually request that they have the gun in hand to make the brass case a true fit as the case which can be pushed into the chamber might still have excessive head thickness thereby preventing the breech closure.
G4me
