I’m looking for an 18-1/2” barrel (12 ga with screw in chokes) for my Remington 870 Magnun. . Is Carlsons the only company that makes this? Is the quality of Carlsons (made in Turkey) up to par?

I have a Carlson's 24" barrel. I think the quality is very good. If I was in the market for a barrel like the one you are looking for I would not hesitate for a minute.

The finish of my barrel is a matte finish that seems to match well with my 870 Express, it would not match well with a Wingmaster.

I bought the 18-1/2" Carlson's barrel last winter and find it a better, snugger fit on my 870 Wingmaster receivers than any of the Remington barrels that I have. The matte finish isn't as coarse as the 870 Express, but not a gloss blue like a Wingmaster. They are chrome lined, so a plus for shooting other than lead. I would not hesitate to recommend this barrel.

The current factory 18.5” barrel are thick enough to accept Rem-choke threading. We (Rose Action Sports https://roseactionsports.com/contact-ras ) have threaded hundreds of them. If you already have a barrel that might be an economical option for you.