ORIGINAL: UncleNorby
My guess is they want to avoid any liability associated with the metric nipple compromising the threads in the plug, so they want barrels returned for replacement of the plug. If it were me, I'd just look at the threads asnd substitute the correct nipple if all looked well.
That's what I did. I went to the local hardware store and bought some red hot nipples and screwed them in. My threads were fine so I am good to go. I just thought I would post the info in case some one had a barrel and wasn't aware of the situation