I've used a local machine shop here in town. Told him what I wanted and how the crown needed to be cared, turn slowly with as little stress as possible and never had any problems. It cost me $40.00
The last one I had done was a 24" 204 Ruger bull barrel from DPMS. I had it turned to .700 behind the gas block and .600 in front, gun shot just as good as it did before but weighed a pound lighter. I bead blasted it as it was SS but parkerizing isn't exactly black magic and the solution runs about $40.00 a gallon.
ADCO does alot of contouring and cutting as a mail order service but they just use their local machine shop.
edit for their link
http://www.adcofirearms.com/shopservices/ you can send them the upper and tell em what you want done.
