Thank you for correcting that, although if you define “platform” as a “modular” or configurable gun, then the AR almost certainly would rule.
the rolling block would come close. Barrel changes could be made pretty easily. They were built into the 1920s.
The rolling block was a beautiful design (they are still in production) and it has surprised me that they haven’t sold better as a single shot.
I would love to see one made by H&R if they decide to bring that company back into production. Very simple to make, they probably could be priced at about 400 or a bit more, like the Buffalo classic.