do you get scratches on the brass when you cycle it? are you using factory ammo or reloads? I had very similar issues, but it wasn't headspace.... It was off-center with heavy chatter,
[img width=416 height=450]http://img713.imageshack.us/img713/5466/6mmchamber8.jpg[/img]
but I had most issues with handloads, when I used once fired brass from the same chamber, it was totally deforming perfect brand new Lapua brass creating physical locks throughout the body....

[img width=561 height=450]http://img191.imageshack.us/img191/4886/6mmnorma5.jpg[/img]

but that's me and my chamber issues... but before go that far, look inside of the receiver to see if scope base screws sticking out, maybe they are to long, also check front action screw, if nothing noticeable get candle and light it up, then get that boltface in the flame and cover it in the soot, cycle that bolt without brass in the chamber, then remove it, any tight spots would show on the bolt-face or around chamber-receiver area, here you can actually see where boltface scratched chamber from 2 o'clock to 6 and chamber is off-center to the bore...

[img width=600 height=375]http://img835.imageshack.us/img835/5607/6mmchamber1.jpg[/img]

as a last step get soot on the brass, make sure its not live ammo, you don't want to put live ammo in the open flame. with candle soot cover neck, shoulders and some of the body, then carefully insert it in the chamber then close and open bolt, that would leave marks where brass contacted the chamber. If you can make some pictures... Just like this one:
[img width=600 height=357]http://img684.imageshack.us/img684/3968/6mmnorma17.jpg[/img]