We have run across the differences in safety
factors before on CF. Mostly with chain grades but also with shackles. So to help with understanding we can do some math.
Crosby G-209 shackles are forged, quenched and tempered (presumed to be super carbon) with alloy pins. Their ultimate strength (us) is 6 times their working load limit (wll).
Crosby G-209A shackles are forged, quenched and tempered fully alloy steel
shackles. Their untimate strength is 4.5 times their working load limit.
The numbers are in metric tons (2200 lbs) BTW.
Ah, did you see the mis-direction? Above you will note that the ultimate strength is X times the working load limit. But that is not really true.
The working load limit is actually 1/X of the ultimate strength.
In other words we do not go up to the US we go down to the working load limit. The reason G-209A has a factor of 4.5 and G-209 has a factor of 6 is that the steel
of the G-209A is much stronger. Looking at it as US as a factor of WLL gives us an incorrect understanding.
Off to some numbers.
Just looking at the numbers and we can see that in 3/8" the G-209A has twice the WLL and 50% greater US than the G-209. A total win here. Almost as good in the 7/16 too.
But the real "shocker" is when you compare the 7/16" G209 to the 3/8" G-209A. The G-209A is smaller but has a WLL that is 25% larger and a US that is the same.
You could go down one size in shackle size.... But of course I like shackles to NOT be the weak link in the anchor "chain" so I use the largest shackles that will fit (in G-209A).
Both the G-209 and the G-209A are proof tested to 2 X the WLL. The spec says that the shackle will not have any cracks or deformities that would prevent the pin from being removed after loading to the proof load. (The pin must be able to be removed by hand after the first turn.)
The point I wanted to make here is that both the G-209 and G-209A shackles will deform somewhere between the proof load and the US (ultimate strength). See the image.
The G-209 is more likely to deform closer to its WLL than the G-209A due to the "softer" steel. Some think that this is a feature.
Of course we do size shackles based in their WLL as the WLL is what is important to us.