Bergara in theory do not sell extra barrels. However, in my personal experience (with a sample size of four now), their manufacturing tolerances seem to be good enough that I have been able to swap barrels between actions and there are no issues with headspace.
Rossi do not, EXCEPT for the Rossi Wizard, but these can be hard to come by. I currently have a Rossi Wizard in the cupboard in 270 Win, but suffers from the odd misfire due to light primer strike, depending on the ammo/primers used. The hammer spring is noticeably weaker than the Bergara, my plan is to try and get some slightly stronger springs made for the Rossi's, which should sort this. One I do, I'll be selling as as I have no need for it now.
H&R did do extra barrels, but you were supposed to send the rifle back to them (I think) so that the new barrel could be properly fitted. Its about having the lock-up just right, not too tight, not too loose, and the cartridge headspace correct.
I suspect most of the single shots that easily break down could be made to work, but should have a gunsmith check/adjust them, at which point its possibly not economic?
There is a guy on the forum would has/had a Bergara Apex with a 222 and a 308 barrel, that was their interchangeable one before they stopped making it. Almost the same as the current Bergara single shot, but without the takedown feature. I think you have to knock out the main pin to swap the barrels, which is pretty quick to do. I was going to buy it before he changed his mind, but I think he might be selling it again? I can check...
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