Will a single length of 550-cord really support 550 pounds?
New 550 paracord should hold more than 550 lbs statically suspended. It is rated for 550 lbs but there is a safety margin built in. Age, abrasion, knot quality etc. could affect how much it can hold in practice. Most of its strength comes from the woven outer sheath which is really strong and is rated for 300 lbs. There are seven pairs of inner strands that aren’t that strong individually but contribute to the rest of it. The biggest problem would be breaking it with sudden jerks so you would not be able to drop a 550 lb weight tied with it and expect the cord to catch its fall without breaking.
ISTR reading a review where a guy bought 550-cord and gently set himself into a loop of it and it snapped. I think he weighed somewhere in the mid-200s, the the cord parted not at a knot or loop. Probably was commercial cord and not mil-spec.
This reminds me of another question. I have an Air Force survival manual I picked up when I was in the Civil Air Patrol. It illustrated how to make a gill net out of the inner strands, using the casing to support them. How many feet of cord do parachute shrouds comprise?
Not really an answer to your question, but 7/64" Dyneema is going for 20 cents a foot at Redden Marine. That is roughly twice the price of paracord, but it is supposed to be rated to 1440 lbs.
How does sudden acceleration (such as from a fall) tie in to the rated strength?
My now-ancient recollection is there are / were 24 shroud lines in a typical USAF chute, and they were about 30 ft long from harness to canopy rim. The lines went all the way up to the canopy crown, but cutting them out of the canopy body was difficult, so you’d plan to use the exposed line first for your various survival purposes.
As Shagnasty said, 550 pounds is the static rating. A jerk on the end of the line (:p) may exceed that limit.
Thanks. It’s academic, of course, since I’ll probably never find myself under the silk; certainly not against my will, nor ever be in a survival situation where a parachute is available.
.
As was mentioned, the rating on rope is for a static load. The rule of thumb I learned doing suspension bondage is that for a dynamic load you want your rope to be rated for 10 times the rating you’d use for a static load.
Would you like to know the first thing I thought of when I read this? 
I don’t think there’s a knot, no matter how well tied, that doesn’t reduce the strength of the line it’s tied in,
this from [9],
CMC fnord!
Is there a non-highly-technical explanation for this?
ETA:
And thank you for this 
Tying a knot in the rope results in kinks and bends, which focus the forces on the rope onto a smaller area. The tighter the turns made by the rope in the knot, the weaker the knot is compared to unbent rope. Suppose a rope rated to 100 lbs is made up of ten fibers, each of which can support 10 lbs without breaking. If you lay the rope over the 90 degree edge of a table and hang 100 lbs from it, the fibers on the inside of the bend and the outside of the bend will be differently loaded, to the point that some individual fibers will exceed their 10 lb maximum. Once those fibers part, the load is spread to the remaining fibers, which fail in turn.