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I'm going to build a harness/container for personal use this summer. My plan is to pull apart my Reactor and reverse engineer the harness, with some design updates like austrialpin hardware, hip rings, dual pin, soft cutaway sleeves, etc.
I'm a senior rigger with some experience building harnesses for round canopies, but I haven't built 3 ring risers. It's my understanding that regular para cord isn't strong enough. What is used? and why isn't dacron used?

Google 3-ring cutaway system design. By "paracord", I think you mean Type II (Inner core/outer sheath). The assumed problem arises that it has a larger diameter than type IIa (coreless sheath) that could bind up, preventing a loaded cutaway (which is rare in BASE IMHO). Have fun and think outside the "ring".
Take care,
space

Type IIa is used. But it's mostly selected for it's ability to slide cleanly when the system is cut away under load, which is not really an important criteria in most BASE cutaway situations.

I'd be really curious to see some work done on improving that white loop, because it is the most fragile component of our life support system.

Borrowing it from a similar sport where it doesn't serve as critical a functions seems like a mistake, but we just don't have the resources to really develop an alternative, so we tend to just copy the (very) field tested technologies from other sports.

I don't know whether they do it these days, but years ago I called Relative Workshop (now United Parachute Technologies) and they sent me the drawings and specs upon request.

If you are going to build your own risers, it seems like a no-brainer that building them to the currently approved design specs, if those specs are freely available, is the best place to start.

Also, skydiving rig manufacturers generally sell the raw materials they use and I would be very surprised if UPT doesn't sell everything you need to make risers.

Walt

Don't worry, some manufacturer will probably start using half diameter material to save 0.1g total weight in whatever new harness they're marketing.

They've sort of gone the other way, with most of the manufacturers pushing sewn in risers.

I'm hoping some revisiting of the basic three ring assembly will happen at some point in the future, but I'm not holding my breath.

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This a couple month old thread, but I am just now reading it .
It not really that fragile for the job it is doing, and it just 1 item of the 3-ring 'system' .
Loop/cord integrity be compromised by friction and cut, but structurally is more than adequate for holding 3-ring system that is sewn to spec. and within the tolerance quoted .
Plus in the system, You only have a single bar-tack to each loop and it is under cover for a reason for protection . but the tight weave of the Type IIa cord really complements to the stitching holding it down for long term wear .
You can hang yourself in a hanging harness with cutaway cables pulled from the loops . You can take your little fingers on both hands, pressed against the Loops to keep 3-rings closed under pressure . for the 3-ring design, I think it only takes a minimum of 4 lb. pressure to hold the 3-rings closed @ the small Ring conjugation where the Loop passes threw . And I think the cord rating is a couple hundred Lb's pull strength .
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I was more thinking that it's fragile in terms of potential damage--like abrasion from getting rubbed against something or dragged while packing, or damage if it's caught against something while crawling around on a structure.