There are some factory barrels that truly excel. Have you had the chance to use any OEM barrels that made you think an aftermarket upgrade wasn’t even needed? I’d love to know which platforms you found them on!
I've only seen one, it was a factory Rem. 700 in 7mm-08. A customer brought it to me to check out and bed. Once bedded, it shot so good ( less than .375 at 100yd. ) with factory Rem. ammo that I told him to buy all he could of that lot and enjoy it. No need for a rebarrel or better ammo. The only one I've ever seen with that good a barrel and it's love for factory green box.