Actually SMC makes both heavy and light rings. For hammock use I suspect most of us are talking about the descender rings because they are low cost and strong enough for properly hung hammocks. Especially with a garda hitch. They are roughly 1/4 in rolled from a flat stamping so they can be hollow and will have a crack around the rim that is the rolled edge. They are rated for around 3000 lbs. Omega Pacific and SMC also make heavier forged rings that cost more and hold more weight. The SMC rigging/descender is rated at ~7000 lbs but is smaller with a 1.7 in OD and 1.1 in inside. That makes it tight for 1 inch webbing in a hitch.