JPK:

Yeah, I would think the chances are pretty good that it wouldn't require re-reg, and .450 beats the .458 all hollow in a DR. I would do the conversion without a second thought. Sounds like a real nice rifle. I'll bet its pretty.

Mick:

That is pretty much the way I've seen it done as well. From the photos I've seen of the process, it looks to me like it is done the same way when building a new rifle - regulation is done after the ribs are soldered on.
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