Aluminum tubes are really stiff - think of an aluminum bicycle frame, which are much stiffer than steel, titanium and most carbon fiber ones. I'm clearly no metallurgist or engineer, but I'd be really surprised if it wasn't only the barrel flexing. It's pretty easy for me to flex/deflect the barrel on my MK18 LW. The primary reason FF tubes/rails exist is to eliminate the flexing/deflection of the barrel when force is imparted on the forend.

