I switch barrels all the time on rifles with and without nuts. On conventionally fitted barrels, they get screwed on tight and taken out and shot. I have never seen any evidence of changing headspace. I used to change barrels between matches ( LV to HV) with no issues. By the way, when fitting a barrel with a nut, I still have to set it up in the lathe to thread and chamber it. I've not seen that it is any quicker either way. Once a barrel is fitted conventionally, it can be screwed off and on and headspace will be just fine. Now, if one really cranks on a barrel with a minimal shoulder, you might be able to get the headspace too tight. Given an adequate shoulder, wear on the threads will eventually get the barrel turning past register but the shoulder location will likely be unchanged and headspace will also not change. I have one pre-64 Winchester action which gets barrels switched at least once a month and no gauge is required. A "go" gauge was used to establish headspace on each of the half dozen barrels which are fitted to this action and there has been and is no need to use the gauge again. The barrels are just tightened up until the extractor cut is aligned and there you have it. By the way, one barrel which is used on this action is a re-threaded, re-chambered Savage barrel which retains the nut. On this one, the barrel is, again, screwed in until the extractor cut is aligned and the nut is set. Because the extractor cut is, necessarily, a bit wider than the extractor, headspace can vary by a thou or two so I do employ the go gauge with it but don't have to with the others.
I have, on occasion, used a nut where the barrel shank was small enough that I didn't think it provided enough shoulder for positive fitting. Again, these were fitted and chambered conventionally (using a gauge, of course) but, subsequent to this, the barrel was just screwed in hand tight and locked with the nut. Headspace was established by the shoulder. I don't know how many barrels of both types you have fitted and chambered and swapped back and forth but I suspect you might need some more experience before being too critical. GD
