The Kerr Sling was adjustable as was the later M1907, but were used for marksmanship - not for combat.
Formal Marksmanship 'training' more or less starts in the 1880s - 'after' the military had enough money to authorize the ammunition - prior to that, some units never fired a shot in their training, simply because the ammunition allocation wasn't there.
Post-1898, the easily-adjustable Kerr 'No-Buckl' sling showed up and some units did have that - most did not - after the Model 1907 sling - still in use today - the various positions were figured out, tested and trained on - but when the rifle was actually needed, there simply wasn't time.
The later cotton M1 sling could be used - and was - but again - not in combat, and for the above reason.
Position shooting is wildly different from combat shooting - time is your enemy, and it takes far too much of it to waste employing what worked for you on the Qualification Range.
In my experience as a career Infantry soldier, the sling was only used as a carrying strap - with most of us just taping the swivels against noise, and leaving it in the Base Camp - if you had your weapon in hand - you could put it to good use quickly.
I suspect that the real question is 'Can I Use It In C&WAS?'
The answer's 'no'.
Scouts Out!