I'm not too worried about the bolts loosening off, I'm more afraid that with hard and continued use that the metal that is remaining on the buffer tube, where the threads were removed, will eventually become so stressed that it would start to widen the holes where the bolts are placed. This would eventually lead to the thin metal of the buffer tube becoming so weak that the metal would just fail.
The horizontal bar/stock retaining screw combo would be able to take the stress of the stock being battered around and if anything would prevent the bolts from having to take all the stress - if nothing else then it would just be considered overkill, but would make me feel better about knowing that my stock isn't going to rip out when I go to shoulder or sling the LMG.
I've fielded the LMG now a few times, it's been slung (to pull a secondary) fairly frequently and battered around, so far nothing has loosened off (I have removed the stock a few times to ensure that the metal of the buffer tube hasn't started to show signs of stress yet). I'll be fielding it at Blacksheep @ Graylings this May, so I'll know for sure at the end of the game if the stock can take the stress with just the 2 bolts or if the retention bar is going to be required. I'll report back
