Full length tubes need to be fit ...by a competent shop ...like Briley in Texas ( for Briley tubes ).
Often - when you tube a gun like a 12ga ...you'll need to change the Inertia triggers (typical in Browning O/U's ) to mechanical triggers for sub gagues like the 28ga and .410.
The ideal set-up is to go with a carrier barrel ( that carry's the tubes / it cannot be shot on its own - it can only be shot with the tubes installed). But the idea is then you have a 12ga barrel - and the carrier barrel with a set of tubes in it, is the same weight as the 12ga barrel ....making shooting all 4 gagues identical.
If you don't go with a carrier barrel - then you have an issue / when you tube a 12ga barrel - the tube sets for 20ga say ...add about 10 oz to the weight of the gun / and it changes the feel and swing characteristics a lot.
That is partly why some guys have a 12ga tubed....and then they shoot the 20ga tube set in all 20ga and 12 ga events --- because transitioning back and forth to the 12ga stand alone vs 12ga with the 20ga tubes in it is difficult in competition.
In my stand alone guns ....12ga weighs about 8 1/2 lbs ....20, 28 and .410 weigh about 7 1/2 lbs ....so I add 8oz under forend and 8oz inside stock ...to get the 3 light guns up to the same spec as the 12ga. Keep balance point the same ...keep the feel the same. But I still have 4 gun cases to load --to go to a tournament...and 4 receivers and 4 slightly different triggers ( they are all about the same...breaking right around 4.25 lbs ...but not quite as good as a stand alone carrier barrel and tube sets.
If you had a pair of Browning Citori XS Skeet guns ( 2 guns will cost you about $6,000 ) ....you could send both guns to Briley ...they would bore out one barrel and turn it into a carrier barrel ....and then fit both barrels to one receiver. Make the trigger adjustments to both receivers ...and fit the tubes ...and buy the chokes, etc ...and a case to lug it all around in....( for about $ 5,000 I think ) - if you provide the guns and both barrels - and you're into it for about $ 11,000.
From gun mfg's like Kolar and Krieghoff ...and proabably Perazzi and Blaser -- you can buy guns with carrier barrels and 12ga barrels - and have either Kolar or Briley tube sets fit for the guns...( prices for these guns with all of that included - are $ 15,000 - $ 18,000 probably ) if you stick with fairly standard configurations - if you go to upgraded wood, engraving, etc you can easily get up to $ 25 K.
If you just want to send a 12ga to Briley - and have them do the trigger, make and fit the tubes, some chokes to fit sub-gagues ..a case for tubes,etc ... its $ 3,000 - $ 5,000 probably..../but you have the weight issue I discussed above.
The 4 stand alone Citori XS Skeet models, I have ...retail new today for around $ 12,000 ... add another $ 500 per gun for choke tube sets ( so $2K more ) ....and a good hard case for each ...$500 ....you're at about $ 15K.