Ported Barrels and Ported Interchangeable Chokes

Ported barrels are permitted, provided they do not extend
back further than 20 cm as measured from the end of the
muzzle.

Ported interchangeable chokes are permitted, provided their
porting plus any barrel porting does not exceed 20 cm as
measured from the muzzle end of the interchangeable fitted
choke.