I had the same problem with my 400, half the time the front of the pellet was sheered off and the other half the pellet would spin and get bound up in the transfer arm or barrel. Unfortunately I had to file down the piece of barrel that was protruding into the breech block. The screw that holds the barrel to the receiver (accessed through the top of the block) sits in a countersunk hole. When you back-off the screw and move the barrel to the desired position and re-tighten the screw, the barrel moves back to its original spot. Moving the loading transfer arm forward or back will not fix the problem either. The thickness of the arm is approx the thickness of a pellet (see other articles on which pellets will fit this model) so if the arm is too far forward either the arm itself will get bound up on the barrel or the pellet will. If the arm is too far back the loading pellet gets pushed through the arm hole (will still bind on the barrel) and the front of the next pellet enters the rear of the hole creating another bind. The barrel on my particular model extended almost to the back of the block and the breech section was notched out. I marked on the barrel where I needed it cut too, removed the barrel and started filing and sanding a little at a time until the fitment was perfect. Now the gun performs flawlessly. Let me know if you need pics.
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