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Well the Spectre sling loop spreads the force. The QD cup relies on a single bolt and applies force to the stock. I'm not worried about the bolt/nut but more the stock - either being compressed/rubbed by the QD cup or being bent. It's plastic after all. The Spectre sling loop can deform and stretch a bit under force, plus there's the sling itself.

When I test fit the QD cup at the rear most bolt by the sling loop, it was already rubbing into the plastic stock. I guess if you use a really long bolt and have the QD cup resting on the nut it would work. I'll think about it a bit more but I'll most likely stick with the Spectre as it is the simplest solution.
 

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Bolts came in and the 40mm are just ever slightly too long so I ground them down with a Dremel, about 1-ish mm for the bolt next to the ejection port and 2-ish mm for the bolt near the slot. I probably could have just left them at 40mm but the bolt tip that protruded past the threads made me a little uneasy, especially for the cup near the slot. The QD still functioned and snap in but I did notice a hint of hesitation, push back. Once I ground the tips to be flush with the cup, both felt much better.

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(left to right) 40mm, 39mm, 38mm, 35mm (the stock bolt)

The longer bolts that fully grabs all the threads and the cup sitting on the nut is definitely the way people should be installing these PMM cups. Only slightly more work but a far better hold IMO. I've got them Loctite and positioned so there's no way for the Loctite to reach any of plastic as it cures, I'll add a dab of Loctite over the bare cut part of the bolt afterwards to seal that section from possible corrosion.

Some of the test fit process pictures:
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