Gentleman I recently bought an ar15 clone from an estate sale which appears to be a parts gun. Its based on a DPMS cast receiver and has an A2 upper, only the barrel is slim like on an SP1. The underside of the barrel is marked  C MP C. Does anyone know who made this barrel?

It's a Colt, fairly early barrel.  The chamber is chrome lined; the bore isn't. Used on M16A1's after reports from Viet Nam that they were getting malfunctions w/the M16s from rusted chambers. Later, of course, they went to fully chrome lined chambers AND bores. 1 in 12 twist, so you'll have to stick w/55 grain loads.

Got one of them myself. Just bought it to use on an early Air force model M16 type lookalike [:D]

since when do you find AR's at estate sales?