CamoCop - I don't know if they will fit over a bull barrel. I tried one on one of my rifles just for fun and took it back. I read the instructions and positioned it as recommended, moved it forward and back to find the 'optimium' point but I didn't see any noticable improvement in accuracy.
And it looked like crap on the rifle. SVL (limbsaver) makes some good products but I don't think this is one of them.