I've noticed a similar effect when using a 17" all leather sling. The springiness seemed to add velocity (with light stones especially). I think it could be that the stretched sling gets shorter after the main part of the power-stroke. Similar to when a spinning skater pulls in their arms, they spin faster.

The military-grade paracord I have is very noticeably springy/spongy to me at 28" The longer the sling, the more noticeable it is.
Another idea is that the springiness temporary delays the projectile's  acceleration as the cord stretches, essentially lengthening the power-stroke slightly. That's what I get from the feeling anyway.

Maybe people who prefer natural fibers notice springiness more or something. Or it could be that the properties of paracord, being relatively dense, might create a difference in feeling that people think is stretching. I don’t know.

joe_meadmaker wrote on May 29^th, 2022 at 3:39pm:

Military 550. Proper dense stuff.
The stretching is most noticeable to me when slinging smaller stones at max velocity, where there most acceleration is involved. I suppose it's relatively subtle, but being used to completely unyielding fibers like linen, esparto, dyneema, etc. I can definitely feel the difference.

Here's my two cents on stretch.

Paracord is designed to stretch but should not be apparent when dealing with the loads we have during slinging (even for 5 lb rocks - Joe, you're a beast). When a cord is unloaded, the braid is in a relaxed state. Once some tension is applied, it will strain until the braid tightens. For Paracord, that tightening should occur during pre-rotations.

Any amount of stretch is bad. Stretch of the cords and pouch, implies that work was done onto the sling. Most of the work you impart to the stone occurs at the start of the power stroke (it will take too long to explain why in this post). As a result, a lot of the acceleration occurs at the start of the power stroke too. As the power stroke progresses, the centripetal acceleration will decrease but the angular velocity (and tangential velocity) will not. Therefore the centripetal force Fc = mv²/r will not decrease. So, any stretch that occurs will not shorten until after the stone leaves the pouch.

You might say the stretch lengthened the throwing arm. That would increase power if the acceleration of your arm and not the force available to apply is the limiting factor (which I think is often the case). However, there is an efficiency loss due to friction (which would still increase power if there is sufficient force available). And if you had any observable stretch in the cords, the amount of work you put into that elongation would be way more than just using a slightly longer cord with no stretch.

I could be wrong, but this is my understanding.