Thanks BBill, I sent them a email as the 6mmbr super bolt and extension with the round lugs has disappeared from their website since sunday. I hope they're just out at the moment.
I had an second run ARP bolt in 6.8 years ago. It is thicker compared to the Busmaster (CMT?) bolt that came with the rifle. I didn't shoot it much with the ARP bolt and I wasn't too impressed with it because it used a 5.56 extractor and a modified extractor spring. I didn't have any failures wiyh it on an intermediate gas system Bison Armory barrel. Although I didn't have any failures with the 1st run Bushmaster 6.8 carbine it was originally purchased as.
Anyway fast forward to 2011. I built a 7.62x39 AR15 upper with an Osprey Defense piston kit. The Bolt is from SI Defense and supposedly MPI, carpenter steel and the whole milspec deal. I also run an AA bolt spring and POF roller cam pin on the OPS one piece bolt carrier. I hand cycled it dry for a long time while locating other patrs needed (including the POF cam pin and bolt spring) and had some where on the back of the lugs and cam pin hole. I never shot it DI but soon after installing everything it hand cycled much smoother and as far as I can tell no more wear (parkerizing rubbing off) on the bolt when lubed and fired.
If I was going to entertain the thought of this carbine as being a serious fighting one I would probably get the ARP bolt for piece of mind. But as a range toy and occasional deer rifle the SID bolt is good enough. I am just shooting steel cased Wolf through it and passing a bore snake through every 100 rounds until it jams. So far the bolt seems to be holding up and no signs of hard extraction.
Well, for what it's worth. I have sent several emails to AR15 Performance over the last month+ without any response asking if they are still making the 6MMBR Bolt & Extension since it is no longer shown on their web site. Since the project is basically stopped without that bolt and barrel extension I'll most lilkely go another direction.
i think were going to see a lot of the makers of one-off stuff for AR-15 type rifles start dropping off the map due to the poor economy and slow sales.
hell i found a place that will make barrels to order but the prices on ammo make if difficult to run off a batch of 6.5g type barrels. that other ar place is sitting on a stack of 6.8s and their moving very slow i have been told.