There is a tool that holds the upper in place, it is like a magazine that goes into a vise, than a star like protrusion goes into the barrel where the bolt fits in, that tool will properly hold the upper, and stabilize the barrel. Than a proper size wrench will take off the flash hider. Not trying to rain on the parade, but much can go wrong without the proper tools. Cracked or warped uppers occur. If it's a one time deal take it to a gunsmith, if you plan on doing it a lot buy the magpul tools.