I've had this problem with Benchmades since I started collecting. Not all but a majority of the models I have owned had the non existent sweet spot. I have fallen in and out of interest in Benchmade because of this but after recently getting another 710 that had no sweet spot I decided to really research into it. I ALWAYS see posts from satisfied Benchmade owners and just didn't understand why mine never were up to par. I stumbled upon a post on here by Lennyo that really shed light on the problem.
In a nut shell, the width of the blade + washers was way off compared to the width of the backspacer, causing the liners to not be parallel. This meant if the pivot was tightened down to eliminate play the washers were being pinched some where on their radius bogging down the action. This explains the weird wear pattern on the washers as well as why swapping washers seems to have fixed it.
On my 710 I noticed when I left the handle screws completely loose my 710 would operate as it should. I could dial it in to have zero play but still very smooth. Also when I would tighten the handle screws down I could feel with my thumb on the spine of the handle that the liners were bowing inwards at the screw points. After reading Lennyo's thread my conclusion: the backspacer was undersized compared to the blade+washers stack.
Well I couldnt add more material to the spacer so my only option was to reduce the size of the blade+washers. I used 1000 grit sand paper on glass to sand down the washers, reassembling the knife every now and then to check progress. I ended up with a perfect 710, rock solid pivot with a smooth action. It's not free swinging smooth... yet, but I'd rather have that final stage of breaking in to happen naturally. It's plently smooth as is.
Thought I would share my experience as it really made me dislike BM, although I LOVE their designs and the axis lock, their fit ruined it for me. I am competent enough to fit my own knife so I'm a happy camper now.
