I have a 300 WM with a factory 24" Rem bbl that absolutely drove me crazy. It took a long time to get it to shoot good but eventually it came around and is pretty close to a one hole gun now. So, you will figure this out and learn a lot in the process.
Here are a few more suggestions:
Get the bbl clean as you normally do. Then swab it with the Hoppe's, gently place it muzzle down on a clean white sock for about 6 hours. Push a patch on a jag through it and see what the patch tells you. There are faster bore cleaners, but Hoppes will clean it, it just takes longer. Other good choices are: Wipeout, TM solutions, Butchs, Sweets. I have them all but can thoroughly clean a gun with Hoppes.
Find a way to check your brass lot. I use a Sinclair neck concentricity gauge. I have had bad lots of brass and have seen improvement when trying something else.
Case Gauges & Headspace Tools - Sinclair Case Neck Sorting Tool w/ Indicator
RL-22 powder is excellent in this caliber but mine wouldn't shoot it. I switched to H-4831 SC and had better results. Every gun is different. Others that did well were H-4350 and H-1000. All with Fed 215's. Start low and work up to see what yours likes.
I like stretching every bit of velocity out of my loads but in this one I kept it pretty mild. The gun only weighs 6-3/4 lbs and is used for packing. My loads only run 2950 with the 180 AB. I lose a little velocity but recoil is manageable and the accuracy is much better.
Your bases and rings are fine for now. All you are trying to do is get a good group at 100 yds. Try that bbl break in procedure, at least you'll be able to get an idea of what your bbl is doing. Fouling etc. Keep it cool when you do it.
Good post, your getting a lot of solid advice and all of us will learn from this one.