Remington made the 600 chambered in 6mm Rem (1964-1967), it had an 18½" barrel. Know of several guys here in South Texas that have have been hunting with them since new. They do have a rib on the barrel but it seems like the 6mm goes for about $500 to $600.
You might also find one that has had the rib lugs professionally removed and barrel refinished. Once aftermarket aluminum & steel ribs were available to replace the nylon rib, that modification slowed because of the value that original, updated rifles can bring. These rifles have also been used for builds.
I have one chambered in 35 Rem, only a bit over 3,000 made in this cartridge. Super hunting rifle. Mine is configured with the Burris scout mount that features quick release rings with "zero" return with the rear sight milled into base. The mounts are as hard to find as a 600 chambered in 35 Rem.
My rifle has the aluminum rib, steel trigger guard (blind magazine), and Timney drop in trigger/safety.