Thanks for the help Hookster. I didn't even think about the problem being with the OEM magazine tube barrel bracket and not the handguard. I guess the specs on the magazine tube bracket under the barrel varies since some people say they have fitting issues and some don't. Benelli probably doesn't see it as a problem since the factory handguards are unaffected by this slight size variation since they push against the spring washers inside the bracket and not the bracket itself. I bet most people don't always notice when the barrel isn't fully seated since it is such a small gap. The only reason I noticed is because the receiver area below the feedramp started to get worn and damaged from the shells hitting it instead of the feedramp. This only started happening after I installed the Surefire rail so upon closer inspection, I realized the barrel wasn't fully seated.
I need the Surefire rail for 922r compliance with my US made full length magazine tube and US made follower, so I took a file to the face of the under barrel magazine tube bracket. I slowly removed material from the face of the bracket constantly checking the fit to be sure I wouldn't remove too much and end up with a loose fitting Surefire rail. In the end, I probably removed about 1/32" of material and the barrel is now fully seated with the Surefire rail installed. I finished the bare steel with some cold blue and with the rail installed you can't even tell I filed the bracket. The factory handguards also still fit fine after all the filing.
To the people looking to install a Surefire or other rail system, I'm going to guess that if your Benelli M4 has about a 1/16" gap between the handguard lip and under barrel magazine tube bracket, at the bottom, then you shouldn't have any problems with installing a rail system. However, if the bottom hanguard lip edge is almost right up against the face of the under barrel magazine tube bracket, then you will probably need to do some filing to get a proper fit. If anyone with a Benelli M4 that didn't require modification to install their rail could confirm this by checking the gap between the factory handguards and face of the under barrel magazine tube bracket, let me know what you find.