Breechplug
how loose can the BP be and still be tight enough to not let blow-by get through? Or was the pourpose of loosening the BP to see if there was room for improvement?
Loose BP - I can not even image shooting a gun with a loose BP. Seen way to many bolts and screws give under pressure when they are loose. I always snug mine up to compress the teflon tape. I just do not want the BP working back and forth at all, so I am not a finger tight person or one that backs the plug off a 1/4 which I have seen suggested.
I suppose for a couple of test shots with a milder load of powder you might be able to get some experiments conducted... Think the best thing is try the 'O' ring under the primer if you can find the right size.
Just take your BP to the hardware/automotive parts store and start testing - get some of the correct diameter and in a couple of different thicknesses...