I think it's important to recognize the difference between stretch and creep. Creep is why you have to adjust your brace height when you first make a string. All material has some creep, and flemish strings may elogate based upon the strands and plys mating more tightly together initially. I don't consider creep to be a very compelling problem regardless of material. Once you exercise it out, either using a braced bow or some other method, generally you are done. Stretch is a measure of the strings' inherent elasticity when loaded. I find elasticity to be very undesirable as it robs cast, exaggerates unecessarily handshock and to some extent limits the practical lowest brace height one can use and avoid interference w/ bow arm. A flemished string with excessive twist may have stretch due to the spring like shape of the individual plys, regardless of the material.
Polyester and dacron, the components of artificial sinew, B500 and B50 materials have more stretch than the more modern materials. Linen I think falls somewhere in the middle, but may exceed some of the modern materials depending upon what you are comparing it to. FastFlight is both a product name and a generic term. In the generic sense it refers to a variety of products made from a combination of modern polymers named Vectran, Specta, Hmpe, Dyneema, et.al.. I think the FastFlight proper material was once 100% Spectra, and is today 100% Dyneema. To be honest is very comfusing as there are a variety of product names for the various raw materials and formulations of these, as well as names for the various bow string materal products composed of these raw materials.
If you google or other search engine the topic, you'll find lots of interesting stuff to read. Most importantly in my view is to try to keep an open mind. Among the things you'll find to read on this topic seems to be an exaggerated inertia to try the newer materials, and in my view an unrealistic fear that lower stretch materials will "break your bow," based upon my experience.
Hillbilly I didn't realize there was any difference until I tried some FF material. Big different IMHO. So much so I have come to detest B50 for ANY purpose, most notably as a tiller string. Just my opinion, for what it's worth and some folks disagree. ;-) Although a spool of FF is more money, it is also twice as much material. I think a B50 string costs about 16 cents to make, FF about 22, same number of strands.
I usually use 15 strands of FF and one wrap of #4 nylon serving to fit 5/16" Bohning index nocks. But you can match your serving material thickness and any number of strands to fit your self, or other, nocks.
