This is Marv cutting a piece of titanium tubing to create an expansion chamber for his home-built
The choice of titanium is to be both a spacer for the first expansion chamber and as a liner to protect the aluminum body from the effects of the muzzle blast from a 10.5" barrel and 5.56x45mm ammo.
For the nitpickers, yes he's got an approved Form 1 for both the carbine and this sound snuffer.