The lead sled has always worked well for me during load development. I don't typically use any weight with it to tame the rifle's recoil, but I do like how it takes the human element out of the equation. Once you have found your load and the rifle is grouping as it should, then get off the sled and onto sand bags or a bipod and learn to shoot well with both.
As a general rule, If I have a decent rifle shooting 2-3 MOA groups, I tend to look first at the barrel and load. With 6 MOA groups , I tend to look at the scope and mounts first. You say everything appears tight, but you can't check the base when the scope is sitting in the rings. Nightforce is a great scope, but even they can have a problem. After checking all your screws/mounts - remount the scope and try it again. If the groups are still 6 MOA, then find a buddy with a 34mm scope body and swap out the scopes. This will quickly tell you if the scope was your problem.
After the scope/mount has been removed from the equation, start looking at the barrel & stock. Check the bedding, look for signs of tooling in both the crown and throat area. Snug down the action to specs. Take a closer look at your brass for addl. clues. Finally, try different ammo.
The final point I would make is not to spend too much time and ammo on trying to find a solution. Your frustration level will rise and barrel life will diminish. Plus you will spend a lot of dollars. Look for the obvious culprits that don't cost much to research. If groups don't improve, send it back to manufacturer and have them look at it.
