I took a barrel off a bsa once and found a gasket made of thick polythene andrew.h.

email (option): warbikes@gmail.com

I use clear silicone on the barrel base gasket...but it's really just a question of preference for a particular sealant. I also use silicone on the tappet plate, sump plate, crankcase to primary inner primary gasket, crankcase joint, clutch cover gasket and footrest and primary outer corks.
I use a smear of light grease on the oil pump gasket and timing cover, both of which have oilways through them which could be blocked by 'stray' silicone..Ian

email (option): Ian Wright