You know a little known fun fact is that the lack of sling mounts are, in a sense, quite... "screen accurate"
*dramatic music*The 10-hole Hero, the rifle most people consider the epitome of what Pulse rifles should strive to be, had NO sling mounts (the rest did, though. Just saying precedent exists).
Front sling was clipped to the barrel vent.
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20150404_123512.jpg [ 160.26 KiB | Viewed 7879 times ]
Rear connector was clipped to one of the stock holes closer to the shroud.
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Rear sling mount.jpg [ 82.57 KiB | Viewed 7877 times ]
(I dont have a clear image of the actual clip, but it is clearly not attached to the rear butt plate. One of the stock holes seems logical)
So back to the Matrix:
With the stock locked in a medium-extended position, the sling on the stock near the shroud is probably pretty stable.
However the barrel vent is a gamble for strength. The vent has about a 3/8" metal tab that holds it to the grenade launcher and the vent doesn't actually contact anything else. It isn't much, but it is better than trying to attach anything to the 'triangle', which is all plastic and hollow (plus filled with the sensors).