rubber or other insert in barrel bore for vibration

My son purchased this mori seiki lathe last fall

Just got it up and running.

He plans to profile and chamber rifle barrels for me. He had heard about inserting rubber or silicone in the barrel bore to help with vibration issues. The cutting will get a long ways from any support and this is a likely issue.

Can anyone comment on this idea or have another suggestion for working on very long L/D rounds in a CNC lathe?