Injun,
I didn't mean it to sound like the change to 209 priming had any effect on blow-by... I was referring to the hollowed out cavity in the breech plug itself visible in the third photo.
With the initial light-off taking place within the end of the plug rather than ahead of it and without that large flat "piston" for the expanding gasses to push against, much less (virtually nil) fouling gets past the breech plug into the standing breech area.
The pressure upon ignition of the expanding gasses pressing outward against the interior walls of the hollow end of the breech plug perhaps helps to form a more effective gas seal as well... much like the brass case of a center fire cartridge does. (
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