Melonite treated ( nitride treatment). It surface hardens the metal without changing the dimensions so it is easier than chroming the bore (which changes the diameter depending on the buildup) and nearly as good.

Melonite treated

Hi Vint, It's a Melonite treated barrel lining. MELONITE is a thermochemical treatment for improving surface properties of metal parts. It exhibits predictable and repeatable results in the treating of low and medium carbon steels, alloy steels, stainless and austenitic steels, tool and die steels, cast and sintered iron.

Melonite treated barrel lining article to Melonite vs chrome cobratac.com/blogs/news/8006929-nitride-melonite-coating-arguably-better-than-chrome-lined-nitride-vs-chrome-comparison

It has a Melonite treated barrel lining Vint.