I've done a little bit of barrel swapping (No1) and found the first stumbling block you can face is stretched threads in both the receivers and the pull off barrels. Some combinations just will not index without shims, and then, unless the receiver has very little wear, you can struggle to find a bolt head to get to spec.
Starting with new made barrels should about halve that problem, but I'm with Bruce in Oz WRT needing some way to check the receiver for stretch and locking lugs wear. If there is an inspection bolt or two among the members here, I wonder if a suitable gauge could be made from the measurements.

Am I reading too much into this?