Ok, I am probably going to stirr things up with this post, but after seeing some near misses: people unable to release their shackle. I thought that I should give my opinion on this.
Snap Shackles are truely not designed to be released under force.
I work (Mechanical Engineer) in an environment where we use eyebolts spring hooks shackles etc and deal with a lot of safety issues. I have had to design some of these parts for unusual set ups. THEY ARE NOT MADE TO RELEASE UNDER LOAD!!! The rating on them is for lifting or securing, but not when a load is being applied. In addition to this, they are being used in an environment with high levels of corrosion (even with Stainless Steel Set ups) as well as unwelcomed particles such as sand or dirt which can only but encourage any kind of malfunction to the opening of the device.
Also, bear in mind that if the shackle does not open, it will definitely not break even after tossing you about. The typical Factor of Safety on lifting equipment is 5:1. Meaning that if your shackle is rated to 2000 lbs it could take up to 10000 lbs to actually break (Load at which the testing actually caused failure in the part). Therefore if it does not open you are STUCK!!!
This should really open up the eyes to everyone out there using these devices. If you are not convinced by this, speak to some of the rigging companies out there and they will confirm this information.
I the new set ups with Pins (WIPIKA, North '03) and locking "finger" are far safer and still can be improved.
Ride Safely!