Yes it is bondo. Bondo is a great filler adhesive. I glued the barrel tube to the mantle and then began fitting it against the tilt mech on the HL turret. I had to oblong the hole in that part lower to get the mantle to line up on the turret, then I tacked it with CA let it dry, and filled the square cavity with bondo. I need it very strong, I have a baby that will probably get his hands on it.