tuning this varmint rifle-wide open for suggestions

WW -
Oops ! Lost the last couple of sentences......

Again, no chrono data to share. I didn't really need it for my application.
Once I find my preferred load... I'll check it w/ my best friends chrono....
located 3 States Eas.....when the planets line up.
Not overly concerned, as DEEP 6 case capacity puts it in the 6-47, 6XC; 6 Creed zone.

With regards,
357Mag

When you say it will not group, what are the group sizes? I know it is a Hart match, but have you slugged the barrel and miked? Are you comfortable with the smith who matched the threads to the receiver? As others have stated, bedding issues, over stressed, etc?

Also, try another proven scope and check bases, rings etc, for I had a good BR scope go south once, and chasing the inconsistent groups drove me crazy until I changed it out.

Are you absolutely certain the crown is smooth? I'd also consider scoping the whole barrel. I know Hart is great, but anybody can screw up now and then.

I sure ain't in your accuracy world as I have very, very little comp shooting under my belt. I congratulate you on your significant accomplishments!
I do, however, reload and have a few custom rifles. One is an AR in 6.5 Grendel. I could never get both accuracy and speed the factory ammo has. With speed, accuracy wasn't there. With accuracy, speed wasn't. Could not make it work so I shoot factory.
I tell you that for no particular reason. Just FYI. I did have two 6mms that I enjoyed reloading; the 6mmRem A.I. and a .243 with 12 twists. Both loved the Sierra 85 gr.hpbt.
Now, having said that, I will say I like the KIS (no second s) method to find why a good rig isn't doing what you want it to do. I am a proponent of using the best mounts, great scope, and mounted properly. I also use a bubble level which really helps at distant (past 100) targets. My Kis method might want you to examine the chamber and especially the tight neck. Bore scope the lands and groves. Check the crown. The heavy barrel probably should be full bedded. Check action screws. You probably already did all this. If all checks out a-ok; I donno. Maybe re-chambering is in order. What you have should work! But there's probably that one little something keeping it from it.....

I had a similar situation and tried every load in the book but couldn't get a group. Went through 3 different scopes as well trying to figure it out, all scopes were high end off of other guns but I was beating my head off the wall that bad! Lol, anyways I found an unrelated article about torque tuning the stock on a savage forum. I read it and thought why not give it a try, I back by action screws out and started at 15 in lbs, not a lot I know but I had to start somewhere, groups were garbage still. Tightened up the action screw in front 5 inlb increments and worked with the tightest load I'd had so far for testing just to see what it did. Well when I hit the 40 inlb mark I started seeing a vast improvement, 48inlb and it was doing a world of difference better. Moved to the back screw and in 5 inlb increments did the samething. I called the manufacturer before applying the torque and they said 40inlb max but they can handle more, they wouldnt give me a spec on more lol. Anyways I would have never believed anyone that torque tuning would make a 2" group at 100 yards come down to 1/2" but after seeing it with my own eyes I'm a believer now. Dont know if this will help or not but maybe give it a try!

"I have about 500 rounds of brass that should fit this rifle so I had the tight neck specs removed by a thoroughly competent gunsmith."

I'm betting that you'll never know if this created the problem but I'm betting it did. I'm also betting you'll eventually rebarrel this rifle and your problems will then disappear.

Even "thoroughly competent gunsmiths" make mistakes just like everyone else.