Well after being excited about my new magpul MOE for-end coming in the mail, I went to install it and the CASTLE NUT of the old fore end will not budge one single millimeter. I took a hairdryer to it and still does not work. The model i have is the REMINGTON 870 EXPRESS MAGNUM. I noticed that there are dimples in the tub (pictured below). You can see that rectangular slit, there are 3 of them. My guess is that something dips into those, possibly the castle nut. The magpul foreend says it fits standard 12 GA 870 shotguns. No details on the magnum or any other model. Please help! Thanks!