I do not believe that full lenght tubes are going to address your desire to protect the very end of your gun's muzzle. These tubes end about 1/2" from the point you are trying to protect. I have 2 sets of 12 gauge to 20 gauge Brileys that are for 28" barrel guns. Both sets have a total lenght of 27.5".
The MWT you quote for your gun should be suffient for shells like RSTs.
Bob Jurewicz