Petzl M37 SL Omni Lock Carabiner

Half-moon maillons both aluminum and stainless steel have a tendency to become racked over time and can be difficult (and some times impossible) to open. I have had students have new aluminum ones weld itself closed so hard it could not be opened with it clamped in a vise and using a wrench. The Omni-Lock is an excellent solution to this issue. I prefer the manual lock over the auto-lock as much of my caving is in pretty bad mud and the auto-lock can become jammed, but I recommend the auto-lock for new cavers until they become proficient enough to be able to make their own decision.