Reducing barrel diameter from 16mm lw

i've just machined a lw barrel(16mm) to fit into a 177 marauder breech(11mm), got the seal groove etc done and it is shooting well bare and loud.

i would like to reduce the barrel diameter to be able to reuse the stock shroud,

any idea what a safe diameter to reduce the barrel to?

i definitely won't go back to the puny 11mm.

whatever diameter used will require making a new air stripper also to fit the muzzle and the oring which the shroud rides on.

thanks,
 
made the shroud mount to fit 13mm and only machined down the end of the barrel.

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