Will iosso and nylon brush ruin barrel?

I agree with that and didn't say that at all (and neither did the other guy). I only said that your example of 416 can be hardened and tempered to different hardnesses. Not all 416 is the same hardness or quality. Not saying that they do but I could totally see a low end rimfire manufacturer using the cheapest quality steel that they advertise and have it tempered to a lower hardness for less tool wear.
I know of and have drawings of other manufacturers barrels where even on a centerfire the RC spec they use can be as much as a full 10 points lower than what we spec. Why they would do that I have no clue.

I think a lot of it goes back to quality and costs. Hell I know one gun maker who doesn't even bore ream the barrel after drilling. They drill the blank and pull the button thru and they're done with it.
 
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