It may not be the base (flat vs. BT). When I loaded for the 22-250, I found (through personal load development and a lot of internet reading) the Sierra 52gr. HPBT (Yes, it is a boat tail) gave the best results. I believe I loaded 'em over Varget.
I would not recommend the use of the lead sled. That device doesn't let your rifle give the barrel it's true harmonics because it's to restrictive. (MHO) Just make sure you have a solid front rest, and the same for the rest on the back. If your buttstock is not resting on a solid sandbag, or leather bean-bag, then your groups will open up...or not be consistent.
Dave
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