-Steiner is done by Burris in Greely , CO
-Kahles is done by Swaro USA. I think its in RI. No idea about shipping.
-S&B I think shut down their US service center? Jerry used to be the man but I think he moved on.
-Vortex is handled in WI. They are among the fastest and easiest to deal with. Pay for Shipping Both ways.
-ZCO while made in Austria has a service center in ID (Ironically close to NF). They pay for shipping both ways.
-Tangents have to go to Armament in Canada, but they pay for it both ways as well. Typically a week turn around time. TT and Vortex are the fastest in my experience.
-Leupold are done in Oregon but their service has gone to shit since covid and are a pain in the ass to warranty through.
-March have to go back to Japan, But there is a rumor they are building out a US service center, which is a huge win!
-NF is handled in Idaho, but experiences may very. Have heard very good to having to sue them to repair/replace optics. Luckily they are robust and rarely need service.
-Minox used to have a good US service center but they are among the worst and slowest. Assume a broken scope will take 6-8 months for repair/replacement.
Those are the ones I have experience with.