I think someone mentioned this but I couldn't find the post. 2 of the barrels I just got happen to be on the recall list. here is the info from their website. They also have a precautionary recall listed in small print at the bottom of the page for other barrels . If you have any drop-in Green Mountain barrels for your gun you might want to check the serial number. They don't seem to be contacting anyone, I just happened to be on their site looking for another barrel. Here's the info

Notice to owners of our Black Powder "Drop-In" barrels: There is a possibility that the nipple on your rifle is defective. Metric threaded nipples were inadvertently installed in some barrels instead of the proper 1/4-28 threaded nipples. The serial number range in question if for percussion barrels # 030672 - 031352. Do not fire this rifle. Please contact us at 603-447-1095 x18 or [email protected] to arrange return shipping. The barrels will be inspected and if found compliant returned. If they are discrepant the breach plugs will need to be replaced so bear with us as this may take a while to complete. We apologize for the inconvenience.

This is the info on the precautionary recall:
Product Recall Notice Green Mountain needs to do a precautionary recall on some of our Thompson Center drop-in barrels. If you have a barrel in the following Serial Number range 026220-027303 please contact Kerry Smith at (603) 447-1095 x18 to make arrangements to have your barrel returned for inspection. Thank you and we are sorry for any inconvenience this may cause.

Well I checked all of mine and I am fine.. thanks for the heads up. This happened once before a few years ago.

Mine's earlier than that. I'm OK

The .54 Renegade barrel I got a couple of months ago had a metric nipple screwed into the 1/4x28 breech plug. I didn't realize it until after my first shoot of about twenty rounds. When I went to remove the nipple to clean the gun the nipple got "wobbly" in the threads after about a half turn out. It just didn't feel right so I check the threads and found the nipple was metric. Fortunately, the threads in the plug were fine and a proper 1/4x28 nipple seated properly and tight. I called GM and almost had to force them to take down the serial number of my barrel. They posted the recall about three weeks later.

chetmarks

thanks for the heads-up I checked all of mine - I am good...

I do not use their nipples anyway - trash them and go for the short #11 TC nipple.

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