5/16th grade 120 chain

That is lifting chain and is too brittle for shock loads. It will lift more than grade 70 but will snap if you panic stop and your load tries to slide/roll ahead.
We had an oilfield hauler in Alberta that thought that 3/8" gr100 was a good move over using 7/16" gr70. After he lost a mud pump on a nasty hill when he jack-knifed on the mud and slid into the ditch with the jeep and the chains snapped he invested in 1/2" gr70 and scrapped the gr100 even the stuff hanging on the rack that had never been on a load.

 

BULLCRAP BULLCRAP BULLCRAP. We had this argument until we were blue in the face. Go read it it’s on here. Your example is incidental, no way to know a grade 70 would have held where the 100 broke. Grade 100 is required to stretch more than 70.

 

Chain Grading


Newtons per square millimeter, etc