I have a rare problem. My FX Crown MK2, .22 cal, originally came with a 600 mm barrel. I recently bought a .177 cal barrel but it won't go into the gun, only part way. I measured the diameter of the brass transfer port and it's 0.537. The original was 0.531. I own three other FX rifles and some extra barrels and measured all of those which are 0.537 with a couple of exceptions. I checked the bore of the reciever where the barrel is inserted and found a shoulder near the breech end and I'm assuming that the machining tool didn't go all the way through, so the ID of that hole is not consistent. If any of you have any extra .177 and .22 transfer ports (dual preferably) that you want to part with, please PM me. Otherwise, I'm going to have to somehow machine the ones I have to reduce the diameter. I did talk to FX and they said that there was no manufacturing change on these parts, so I guess I got a Friday rifle.