So I was looking for solution for a forward sling mount for my Mossy 590. I wanted the front point forward of the forend, yet not at the end of the mag tube (which is where the sling mount is. I also didn't want a clamp on sling mount bracket.
So... I landed on GG&G's forward QD sling mount (the version I have is the QD socket version). The construction is fantastic, the design is fantastic, it's limited rotation, etc. All good!
EXCEPT the fitment. They advertise this as "Mossberg 590". That's a bit misleading. Upon fitment, it's obvious this is made for the 590a1 (big difference). So the bracket goes on forward of the barrel lug, sandwiched between that lug and the end cap (in my case, a +1 extension since I have a 590 special purpose. NOT an a1 thicker barrel). Because I do not have the thicker a1 barrel, the mount moves side to side a bit. That movement causes the end cap/extension to, consistently, loosen up the cap (the design is great, with ball bearing, and locks in tight... but the movement causes the bearing to continually move to the next notch, loosening the cap). To add to this, all the movement cannot possibly be good for the mag tube threads that it rides over.
I wrote to GG&G asking if this (590) mount is also for non a1 barrels. They never replied (2 weeks now), so I ordered it anyway to try. Well now I know. So I then sent them an email explaining why this is not appropriate a NON a1 barreled 590 and still heard nothing back. Their CS seems to be non existent.
So if you have an a1, I think this mount would be AMAZING. If you have a standard 590... don't waste your money, or plan on modifying it so it's a tighter fit, or you'll have issues.

Rant over... and just some advice to would be buyers.

ETA: the mount is GGG-1428

So I found my solution. I'm not going to use any extra attachment. I'm simply going to drill/tap the underside of the barrel lug and use a short sling swivel stud. This will put the attachment exactly where I want it, and will be rock solid. I already have a few extra Mossberg mag cap studs laying around, and they're the perfect length for the job.  At first I was concerned about the barrel lug strength being compromised, but then found that's how Mossberg used to do it, back in the day, anyway (and the lug has plenty of beef to it) so it shouldn't be an issue.