I can't figure out what happened. The rifle went from my safe to my truck and as far as I know, it was not dropped or anything. The only think I can figure is that it spent the night in the truck and the temps were in the lower 20s. Maybe it got cold and brittle.
Steels often show a transition to brittle behaviour at low temperatures (depending on the alloy), but aluminum alloys generally don't perform much different at cryogenic temperatures compared to room temperature.
Maybe that mount just took a bump, without you noticing.
