I remember seeing Flaig's advertisements from years ago. On some of their rifles and barreled actions, they did claim to use Douglas barrels.

I don't know if they used other than Douglas barrels on some of their other actions and rifles or not.

Seems like I remember they sold complete FN Mauser rifles, made in Belgium, and these might have had factory barrels on them. What factory, I don't know. Maybe FN. I think some of their barreled actions came from Belgium already barreled, also.