This is what a "proper" barrel looks like
http://i264.photobucket.com/albums/i...psac9f2085.jpg
Oh, how I hate the M4 profile barrel! It is just incorrect. Proper barrels are thickest in diameter at the chamber and taper towards the muzzle...not the other way around.
I wanted a service grade Grendel barrel, but they are only available in M4 profile. So, I sent it off to have it re-profiled; the section in front of the FSB is now .600".
Okay, men! If you have an M4 barrel and not as a "clone", it can be fixed; send it off to the shop! :D