https://www.briley.com/c-416-companion-drop-in-tube-sets.aspx
"The advantage of the Companion series is that they are ready-made tubes that are shipped direct to you and do not require you to send in your barrel or gun."

The tubes I had were equipped with a small plastic not quite flush rod at the muzzle end to keep the tube tight.
There will be a significant build-up of powder residue and debris in the bore just past the end of the tube, and the bore will need to cleaned regularly.

https://sites.google.com/a/damascusknowledge.com/www/home