I'll bet you had some burs under the plastic piece. If very tight it could have masked a barrel fitment problem.
Remove the forend, and put the barrel back on the frame and lock it up. Is the barrel loose on the frame when closed? If not, go to the faq's and look for barrel fitting. You may need to shim your barrel.
If its tight, proceed to below.
Then the two screws holding the plastic piece on. Make up a shim from a piece of card board or paper and re install. See if that takes away the slop.
CW