Carabiners can hold 4000 to 6000 pounds, making them strong enough to handle the loads found in normal climbing situations. Military and professional rock climbers use this for fast hookups. Locking Gate biners have a nut which screws over the gate to prevent accidental opening. Aluminum carabiners combine high strength with low weight, a real advantage for climbers. However, aluminum alloys tend to be more brittle than steel.
Steel is more resistant to stress cracks caused by dropping or rough handling.
A dropped biner may not crack at the time, but may become more brittle. With repeated loadings, a crack can develop later in the brittle section. Manufactured are reluctant to give definitive answers on drop height because of the variables involved. A drop, any higher than waist height on a hard surface, should be retired. Use big, heavy locking carabiners.