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Kurtzif you want speed:
1) increase length
2) decrease thickness (cord and pouch drag)
3) decrease mass
if you want pouch control:
1) decrease length
2) increase thickness
3)increase braid tightness
4)use a stiffer material
if the round cords are too stiff when the sling opens and throws your shot off try flat cords of the same thickness or make thinner cords
I haven't seen any benefit from heavier slings, most of the benefits are from the increased thickness not the mass that comes with it. heavier slings will be better with heavy ammo, the reverse is not necessarily true. Instead heavy ammo needs slings of higher pouch control
you can learn how to sling with most slings if not all.
learn to adapt to the timing of each sling don't expect to have the same release timing with a thick short versus a thin long sling