Regards. Bought mine in the late 90's. Manual states the action is timed specifically for Winchester 40gr jacketed HP's, mine runs nicely with Hornady/CCI 30gr V-Max.

16mm rings or base, CZ-527's work. Although they'll serve in a pinch (hint), avoid Millet mangle locks, voice of experience.

https://www.rimfirecentral.com/forums/showpost.php?p=5472261&postcount=2

Reliable vendor, reshipper if not in stock so may take a while if not immediately available:
https://www.optics-trade.eu/us/mounts/shopby/suitable_for_brno-brno_zkm_611.html

You'll need to pull the forearm to clean from the breech.