That much overlap would be a pretty strong joint but, on top of the static friction, pressure from firing will expand the internal tube against the external one and increase the friction between the two pieces. Depending on how tight the fit is between the transverse faces of the two pieces, gas theoretically could get between and exert a force tending to separate the two but I do not think it would be enough to cause movement. A weld between the two pieces would certainly resist this kind of movement and wouldn't hurt anything.
But you do want the transverse joint to be a tight fit.