Theo barrels are usually around 15.78mm o.d, (tolerence on these seems to be around +/- .05mm), the reciever is exactly 16mm, so anything in that range that will provide a nice pushfit will suffice, the closer the better though as the action to barrel transfare is a metal to metal seal.

Best to just turn it down at the breech to a length of 41mm to acomodate the recievers depth, then again at the muzzle to accept the silencer.

The step at the breech for magazine location should be 10mm o.d x 1mm deep, you can copy the blind location hole and transfare port off the other barrel, as its FAC i'm not giveing the dimensions on here.


HTH.