I am thinking about a set of Briley 12 g to 20 g Companion or Sidekick tubes and have some questions. I'm asking here because I'm more likely to get honest answers, good or bad:

Do the Briley tubes handle all of the the pressure of the shotgun shell or does some or all of that pressure still affect the original barrel

Will they work with a gun that has 2.5" chambers
Does it matter if the barrels are Damascus
How much does it affect the pattern in a 12 choked cylinder/open

Any other pros/cons I didn't think about?

Both are of course drop-in; not fitted.
The chambers are high strength stainless steel; the tubes aircraft aluminum. Early on there were reports of the 12 to 20g tubes cracking at the extractor cut-out. Have not seen any reports recently.
Briley at one point & maybe now recommended only standard target loads to be used with the tubes.

Your gun barrel simply acts as a carrier barrel; no pressure transmitted to the damascus tubes. I've seen reports of guys using the tubes with cracked damascus barrels.

The Companion tubes come with flush choke tubes.
The Sidekick 12-20 and 12-28 seem to keep the original choke %; the .410 maybe less so.

I had 12 - 28g Companion tubes which worked fine except as drop-ins they do not extend all the way to the muzzle, and there is a significant deposit of hard burnt power/plastic residue that builds up just past the end of the tube in the bore. That was a problem with the old Savage 410ers, and definitely occurs with the shorter Sidekicks also.
SO they need to be removed after shooting and that ridge scrubbed off.
Addendum: I wonder if one could purchase Briley extended choke tubes that would exit past the muzzle? Might not work if the original barrel is full choke.

re: working in 2 1/2" chambers. Small image but it shows the profile of the external taper 12 to 20g Sidekick, and I believe it is for a 3" chamber. It is possible Briley could custom grind the chamber, at a custom cost. If you talk with Briley please let us know.

I fond that some brands of factory shotshells would stick/expand in the chamber and need to be pushed out.

I received an email response from Briley, both the Companion and Sidekick tubes require a 2 3/4" chamber. Briley stated that if the wall thickness was good they could re-chamber the gun to 2 3/4" at $105 per barrel for both the Companion or Sidekick tubes.

I'm glad I asked, but I'm not going to have the chambers lengthened on either of the guns.