This biner is my permanent belay biner. It's fairly large ("Does your biner hang low? Does it wobble to and fro?..."), so it'll easily accommodate a munter or whatever else you might throw it's way. I only have one large locker on my rack, and this is it. The gate action is somewhat slow (6 spins up) but gravity helps it the other way (4 spins down). No matter the orientation, I've never had an issue with it becoming unlocked, even after an aggressive belay.
