[Lt. Dave]

Close barrel till it latches.

Hold sideways up to bright light.

If any light between the barrel and breech face is visible then barrel lug is too long for your receiver.

This is the first step and will help you determine the next.

Always do this first.

 

[Lt. Dave]

^^^^^^^^^^^^^^^^

This part under the barrel that houses the extractor and pivot. This is the barrel lug.

 

[Lt. Dave]

Don't jump too far ahead.

Do you have a gap between the barrel breech end and the reciever? Did you hold it up sideways to a bright light?

 

[Lt. Dave]

Ok, that makes sense. The reason the barrel catch can't get fully latched is the dimension between the pivot hook and the breech face is a little too long, preventing a flush fit between the back of the barrel and the receiver.

The fix is to either remove a little metal from the pivot hook (at the front of the lug) or remove a little metal from the barrel breech face (at the back end of the barrel). Both will do the job. Removing from the hook won't change headspace. Removing from the breech face will reduce headspace. Have your smith check headspace to make sure it is good before deciding which way to go. It only takes a little bit of removal, go slow and check often.

 

[Lt. Dave]

There can also be a gap in back if the lug is too short. In this case the latch may catch but the barrel will be loose and will move from side to side when twisted by hand. In this case a shim at the front pivot hook would be needed.