I'm guessing that it is the angle the rim is trying to engage the bolt head at.

I'd kinda thought about that, but ruled it out as I believe not a factor for other single-loaders. I rebarreled with the .243 factory barrel immediately after the failed RUM chambering test, and merely sticking a Go gauge partway into the chamber the bolt head was able to capture the rim while locking.

What did your extractor come from? .223 would be too long.

I bought a complete bolt assembly through Gun Shack. Since the bolt head comes stripped, they thoughtfully ordered the missing components at the same time. I presume they got the right ones; IDK if they have a master parts manual or got with Savage to spec the stuff.