I had the same problem. I ended up using penetrating oil, a punch and hammer. The nut had a bunch of shavings in the threads which locked the nut in tightly. I wanted to know which tool they used at the factory to tighten the nut in the first place... I got it out though.
It looks like a (4) hand job but its rteally not. If you lay the gun on the side where you depress the nut and place a little piece of plastic under the button the nut will lay pretty flat with the locking nut released so you can pop it with the hammer. (2) things. The instruction booklet shows a picture in the front of the proper nut rotation. The written instructions may lead you to rotate the wrong way! The picture is correct.
Good luck. Get yourself a Green dot scope... @cabelas... $20 (sale)