Remove the barrel and and clean the inside and outside threads. Look to see if the front scope mount screw is protruding into the threads. Remove and shorten if needed. Using some lubricant try to thread the barrel in again. If it is still tight Measure the threads. If they are over 1.055 take some fine emery and lightly sand the top of the barrel threads. Clean the sanding grit and debris. Try once again and report back.

Edit: Apache beat me to it.