it all depends when you baught your L85.
i dont know when they changed it but if you got an old barrel then you will need 2 new plastic clips to make the precision barrels fit.
i have done this to mine and with the 2 new clips it fits perfectly in the old hop.
madbull, Promethisus and systema are all the same so will all fit into the right clips.
the M16 barrel is slightly longer than the L85 barrel but it still fits OK without cutting it down and the barrel cannot be seen through the flash hider.
if you were lucky enaugh to get a new L85 then all the new barrels should fit without any changes