Sideclips seem to me to be too much trouble to fashion and fit if they serve no real function. On rifles that operate at much higher pressure than shotguns, they would seem to brace the joint of barrels and receiver against the tendency to wear from side to side. They are much closer to the point where force is generated than the barrel lumps are. My guess, and it's only a guess, is that on really good guns they have a function, and on poor ones they're just tailfins.