I shoot the German Mauser 11,15x60R (.43 Mauser'ish)
And the Swedish/Norwegan 10,15x61R (see above)
Both bottleneck's, I use a 3/10" grease cookie in both
And as mentioned: fouling control of some sort is key!
As pressure rises BP will burn cleaner, but only to a certain extent. So duplex is no stairway way to heaven, but it helps.
In the Texan dry heat whiping between shots is almost paramount, but here in shitty cold Denmark i can shoot 40-50 shots without blowing or cleaning in winter time!
I dont see as much fouling as a ss case and that might be because the bottleneck itself restricts "gas-venting", thus keeping the burning pressure up thus fouling down.
When the temp is low and humidity is up you might see a dark orange/red-ish discoloring in a newly fired case. It's called "dragonsblod" and is a rare sulphur bond that indicates good/pure BP burn.
It is a S8 bond:
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