The swings and fixtures at most hardware stores are only rated up to 70 pounds. You should purchase playground grade sets and swings and fixtures from commercial vendors of equipment intended for public playgrounds (schools and parks) for heavier users. Substitution of general hardware for swing hardware will result in fixtures which wear through and break during use.
Light weight metal tubes swings with I-bolt fixtures are generally only suited for individual light weight users (under 70 pounds). For safer use or heavier users, secure swings to sets with tubular metal cross bars using clamps that screw around the tube.
For wooden swing sets, use a commercial grade beam hanger rather than and I bolt. Be sure to use a large washer or plate on the top side of the wooden beam to prevent pull through or compression of the wood when tightening. Use weather proof hardware and bolts (zinc coated or stainless steel) for attachment.
Use welded steel link chain 3/16 or thicker for heavier users. The chain should be weather proof, zinc plated or plastisol coated.
A head shackle with bolt (about $3 each) is the easiest way to attach a swing to the chain. Or use industrial grade S connectors ($1) to connect the swing to the chain for a more secure connection for heavier users. An expensive (over $100) tool is available to close the S link, but it can be closed with a lot of pounding with a hand sledge hammer against a small anvil. You can order swings with pre-attached chain on an S link from Pro Swing Hardware.
For users over 70 pounds, select reinforced swing seats. For adults, use a wider seat. Natural and recycled rubber black seats will ruin clothing. Wet your thumb and see if the color rubs off.
Check connections every spring for tightness, rust, and integrity.