Hi, how are you? Let's see if I can help.

I suggest you take this firearm to a skilled gunsmith who can do this for you, that being said, if you feel comfortable, you can try doing it yourself.

The first thing you need to do is, take apart the assembly so that you can hold the parts in your hands. Then you take the pin that is moved by the thumb lever to disconnect the barrel and very carefully and a little at a time, polish it using 400 grit wet or dry or emory paper. Remember, the point is to polish and NOT to remove metal, however you may need to do a little of both. After this, you can do the same thing to the barrel catch where it connects to this pin. DEFINITELY DO NOT take metal off, just polish.

Hope this helped. Please take a little time to rate my answer, it will be greatly appreciated.

G.C.