So I have a new to me 870 Police Magnum that has lots of external wear but has been shot little. Trying to install the DSF-870 fore-end but the castle nut is locked up. The surefire tool is not helping. Have tried hitting it with a heat gun and putting the Surefire tool in a 6" C clamp for leverage - no dice. Any tips? At this point I might just buy a new fore-end + action bars...