I go along with Larry...use a new case sized in the sizing die you will use for all reloading of this system. Mark it and put it in a plastic tube so it doesn't get dinged up.
Screw the barrel snug against it, run the nut up hand tight, then open the bolt to finish tightening the barrel nut. That way you won't crush the case and change the barrel seating. Also, use a LITTLE antiseize, watch the barrel doesn't turn in with the tightening of the barrel nut and mark the nut, barrel and receiver position so you can come back to it and know things are still copacetic.
I have 2 LA and 2 SA Savage actions right now, each with several barrels covering all the different case head sizes plus 3 other "switch barrel rifles, two Rugers and one Rem and have used switch barrel systems for over 40 years and switching barrels on Savages since the early 60's (back when it was "secret knowledge"'. Nothing easier plus after adjusting the sizing die to the chamber the sizing is minimal and brass lasts a very long time in the smaller calibers.
I loaded and fired one 223 case 50 times, in my 26" HB Sav SA switch barrel with the accuracy load for that barrel. The 50 rounds went into 0.700", most were in less than 0.500" and the ones that went outside the 5 were my fault. It was in conjunction with a comparison test I ran between an RCBS FL die and a Redding FL Bushing. Took me slightly over 2 1/2 days and the case was still going strong using the Redding die, but I was plain worn out walking back and forth between the loading shack and the bench. The same test with the RCBS lasted 37 reloads before the case head separated, so I use Redding FL bushing dies whenever I can. Cases that have a close fit between sizing die and chamber and not loaded to bust a gut a very long time.