It's nylon-covered elastic cord, typically used for bungees. 3/16" is the specific size I designed around, but I'm using 1/8" for the accessories. For further novelty it is offered in a variety of colors.
17 inches overall length (1.1 stryfes)
10kg max load (3 bands)
Top-slide prime (of the plunger and ramrod assembly)
Hardware Kit Price: $30
Assembled Price: $70
Planned File Release Date: November 25th
Current fps avg: 122 (with an unsealed breech)
Hardware kit will be: Barrel, Ramrod Core, Plunger Tube, 123 o-rings, 012 o-rings, elastic cord (1 or 2 sizes), two 13-1/4" length of 10-32 threaded rod, 4-40 screws, 10-32 screws, hex nuts.
The plunger catch surface is way at the back end of it so it won't be compatible with typical compression springs. At a later date I will look at what possible small OD springs could be used that would fit inside of the catch surface portion of the plunger, but that would be an aside.
The elastic cords are really really cheap compared to steel springs and changing the spring load on this thing is as easy as opening the fore-end and taking off or adding another band. And there are alternative options for band types and materials to toy with as well.