Ammo and components availability has improved significantly the past year. I built my own during the summer of 2023. It was a fun project. Built it on a standard military Czech 98 Mauser. Very doable and affordable. Standard action has shorter bolt throw than magnum. Only holds four cartridges which should be plenty for any experienced hunter. I ordered new bottom metal for 416 Remington and it worked fine without any modification. Also ordered a magnum follower for it but I understand many simply use the standard follower for this build (it was a cosmetic issue for me). A good gunsmith may be able to modify the walls of the standard action magazine but I suspect it might be cheaper to order new bottom metal. The only concern would be is the gunsmith capable of modifying the rails and feeding ramp? Tricky but I did it and I'm no gunsmith. We have read on this forum of more than one of these attempted builds messed up and ruined by shade tree "gunsmiths" (I will put myself below that category) screwing up the rails. Opening up the bolt face should be no problem (a local machinist who'd never done any gun work opened mine). Modifying the extractor is also a bit tricky but less worrisome. If first attempt gets messed up, a replacement extractor can be picked up easily to start over.
Building on a magnum CRF action would be pricey. Used donors aren't cheap and new magnum actions are hard to get and very expensive.
I don't care for R8 so this is kind of a moot question for me. I wouldn't own one. However, I think in the end you would find it much more rewarding to have a classic custom gun built for you vs just ordering another factory made barrel for a space age rifle. But that's just me.
Here's the 404 I built. Still need to replace the recoil pad. That old White Line trap pad came on the used stock that I refurbished. I need a bit more LOP so am ordering a thicker red one from NECG.
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