I bought a barreled upper at a pawn shop today Was very clean no scratches.  It is an A1 style upper (non removable carry handle) with a free float tube I've never seen pencil barrel with no bayo lug,  Very lightweight.  the only marking on the upper is right in front of the rear sight.  there is a  C  right above the letters  MB.  On top of the barrel om a row moving from the front sight to the end of the barrel is the letter C   MB  5.56  NATO 1\7.  There are some very small ribs from the front sight base to about and inch before the end of the barrel.  The barrel is 16.5 inches.  Anyone know this.  Sorry no pics my wife has the camera on Vacation.  

It sounds like you have Colt components. Are you sure that the "MB" on the barrel isn't really "MP"?

It is MP not MB on the barrel.  Any Idea on value?  Wondering if I got burnt.  Also I wanted to buy a lower to go with my DPMS target upper and use it on both.  The holes look the same size to the naked eye but I don't really have any idea what i'm looking for.  

Any more info.  Thanks M4 Madness for your help.  What is a good bullet weight for 1in 7

Colt 6520 upper receiver.  It's a light weight A2 carbine that Colt sells.  1/7 twist will handle bullets up to 100 grains.