It probably has some locktite on it.
This method will work if the brute force method gives you grief
Dab some Kroil on the inside of the receiver. Leave over night with the barrel pointed up to the sky. Next day grab your old lady's hair dryer or heat gun (if you have 1). Warm up the screw on the inside. Then with your rachet with the big flat head and extended adptor or it crank off with ease.