I don't know if this is your problem but it might be something to look at.I have an H.S. Precision rifle that was a 7mm Rem Mag and I had it rebarreled to a 300 Win Mag a few years ago. The bolt has a sako extractor and it wasn't working all the time so I sent the bolt back to H.S. to have a new extractor installed. I am not sure if I informed them the rifle was changed to a 300. Problem is extractor isn't working all the time I was thinking I might have a new aftermarket bolt installed on my rifle ? Thoughts please and thank you !
A problem can sometimes occur with custom bbl's and chambers due to the tight specs. The clearance between the extractor and the recess of the bbl where the bolt slides into could be at minimum tolerance. (This is the area at the very outside of the breech where the bolt nose slides into the bbl.) As the Sako extractor swings out when it tries to catch the rim of a case it will impact the side of this recess before capturing the rim. This impedance is enough to cause a failed extraction. The cure can be very simple. Simply take a file and remove a small amount of material on the outside of the extractor, then buff with a wheel. Generally it will not take much and it is easy fix. Sometimes you can put a bit of taper on the outside tip of the extractor and polish it, yielding the same results.
I hope this helps.