Should you match carbon steel barrels with carbon steel gas blocks, and stainless steel barrels with stainless steel gas blocks, so you have closer thermal expansion rates betwwen the two components? Or is this a non-issue?
It’s a non-issue. There are thousands of competition rifles out there with stainless barrels and carbon steel FSB’s that work just fine. I’ve never seen a problem with mine nor with any other match rifle that I’ve been around.
Plus, it’s rare that you will use a stainless barrel for rapid fire since they are mainly used for precision rifles.
No problem with carbon & stainless steels. On barrels like 16" (or shorter with CAR gas systems) having aluminum gas blocks and a steel barrel can cause some problems since the differing expansion rates.