So, here's some thoughts.
A. You must unlock the better slings rather than just a straight purchase
B. It adds to the hold breath mechanic. I.E. the basic sling helps about like the perk to reduce sway and works along side said perk.
C. Three levels of sling/sway reduction. Basic, Practiced, and Marksman.
D. These can be unlocked with any weapon that uses a sling rather than a single weapon class.
E. Leather slings further reduce sway, Synthetic slings are half the weight of leather slings.
F. The Basic slings are equal to tier one of the sway reduction perk, marksman slings are equivalent with tier 3.
At a certain point, with the third sway perk unlocked and a Marksman sling, your sway would be reduced to the same level as the regular tier 3 perk with your breath held without actually holding your breath and basically non-existent with the marksman sling and tier 3 perk during hold breath.
It sounds dumb how I worded it but I never claimed to be smart, anyway, it's OP but to unlock marksman tier slings you'd have to WORK to unlock it no differently that unlocking the .300 from a level one account. It's not much different from actually training to shoot accurately, which should happen naturally through gameplay.
This sounded good in my head but lemme know it something sounds weird