im still a novice here, so feel free to correct or ignore this, but is it not just as simple as bullet stability? the rossi has 1:30 twist, if you are firing 158gr projectiles at subsonic velocity, the barrel twist is not fast enough to stabilise them. you would need to go lighter and/or faster. but interesting the deeper seating made a difference, makes sense that getting rid of some of the empty space in the cartridge would make it more consistent burning as cordite says.
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