Y'all make me doubt myself, so I went looking for answers. What I found was clear as mud.
From FMCSA regs 393.104:
(e) Manufacturing standards for tiedown assemblies. Tiedown assemblies (including chains, wire rope, steel strapping, synthetic webbing, and cordage) and other attachment or fastening devices used to secure articles of cargo to, or in, commercial motor vehicles must conform to the following applicable standards:
An assembly component of . . . Must conform to . . .
(REMOVED BY ME)
(2) Chain National Association of Chain Manufacturers' Welded Steel Chain Specifications, dated September 28, 2005.
(Further down and not copied are specs for straps, rope, and cable.)
So I looked up the NACM specs, and this is the best I can do:
Sorry about the fuzzyness, it was part of a PDF, and I copied it as an image and made it a jpg.
Note that it doesn't say Grade 43 can't be used for securement, but does specifically list G70 for cargo securement. Looks like an attorney's dream...
This law also only applies to commercial motor vehicles, there are probably state laws that apply, but I have an hour into this already. Were it me, I'm not tying anything down with a G30 or G43 chain. If something bad happens and you're using the good stuff, it's one less thing to sweat about. Also, part of my original comment was trying to convey that much of the lower grade chain is cheap overseas crap and the rating stamped on them doesn't mean squat. I've seen supposed G43 5/16" chain snapped by a 700lb ATV. It's C-R-A-P and if you trust your equipment to it, you deserve what happens to it.
Yes, I've broken G70 3/8" chain, but it was chain that was past it's prime already and should have been replaced. I've stopped my tractor dead in its tracks catching a log on a stump more than a few times, and I'll bet the tractive effort of a 10K# farm tractor is close to what a duece will pull, if not more on loose ground.
Use what works for ya, and I hope I at least confused a few people as much as I did myself researching this!
BTW, someone mentioned Amsteel synthetic rope. I've got 150' of it I haven't got to try yet, but it's rated way higher than 1/2" chain, and the whole rope, 2 clevises, bag, and tackle block weigh less than a 16' 3/8" chain does. I'll put up a review of it when I get some use out of it (soon I hope...rain rain go away!)