Is the barrel extension original to the barrel and has it never been removed? I only ask as there have been quite a few used barrels sold lately without the barrel extension and it is very difficult to get another one to time correctly. Is there any side to side play with the pin in its slot in the receiver? If so shim it. If its the original barrel extension that came with the barrel and its never been removed then most likely your front sight base is canted and must be straightened. Take the front sight base off and make sure the gas port is in line with the barrel extension index pin. if it is, you have just confirmed the front sight base is canted. The easist way I can tell you to fix it is to drill the pins hole a little larger and install larger pins. To center it install the barrel in the receiver and the front sight base without its pins. place the barreled upper upside down on a flat surface like a kitchen counter top and try to rock it back and forth. It should rest evenly on all four sight ears when its on straight. Re drill the holes a little larger and you're good to go. I also have straightened a front sight on a FAL by peening the displaced metal back into position. You could always just put a clamp on style front sight base as well.