They 2 are very different beasts. The QD cups incorporate a grove into which the sprung ball bearings in the male portion of the QD slings fit. Once they are engaged the male portion they can turn but not pull out. The Spartan only has a magnet to help locate the bipod male portion and it’s not strong enough to act as a sling attachment. Remember, the standard external Spartan adapter incorporates a standard swing swivel attachment so you can have a sling with a standard at one end and a QD at the other, but it’s probably easier to just use the standards IMHO. Early Spartan adapter did have an extra feature to hold the Spartan 300 bipod in place but I wouldn’t trust that for a sling either.
On an 85, I did fit a Grovetec QD into the butt spacer I made at the bottom of spacer so you could do something similar on the S20 and use the forward side QD cups already fitted. I also fitted a Grovetec cup to the friend side of a Wideland Veil stock (same as the S30 side ones) so that could be a better possibility for underneath if you must.
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