I got my dad's Belgium A-5 20 ga. from his house the other day. He has had this gun before I was born, but I've never seen him shoot it. Total, it's probably had two boxes of shells through it ever. Sitting so long I was going to take it apart and clean it before I took it out to shoot. Problem #1 was finding a dang screwdriver to fit it. Got that taken care of and then problem # 2 was that all of the screws were stuck. I got all of the lock screws and all but 1 tang screw out. However, the last screw will not budge. It is the rear screw just before the stock. Is there anything I can put on it to help it unseize, but not affect the gun. I have the stock off and I've tried spraying it in gun oil and sit to no avail.