You don't let it "drop" into place....this will put dents on the inside-rear of the barrel-jacket and may just keep the barrel-assembly out far enough to make rotation of the jacket-assembly hard..........also, the "Gunner" would slide the barrel in from the rear during barrel-changes .....correct (?).
If the barrel will not fit, measure the piston area and find out why. Carbon-buildup (?)......bent-barrel-jacket or is the barrel-warped/bent slightly (?)......trying to install and MG34/41 (MG34S) barrel-Assembly which is larger in diameter on both ends (?)........nicks or other binding areas on the barrel-jacket and/or barrel-assembly (?)........dented barrel-bearing (?)........any of the rivets or Bipod-Mounting-Pin on the barrel-jacket loose and protruding into the inside area (?)........does another barrel-assembly fit (?). You have quite a few items to check so let us know what you find.
Regards, RichardS in MI.
Blanksguy2001@chartermi.net