... I'm not familiar with the "retaining screw" on top of the receiver.
There are two large Allen socket screws along the top of the receiver, in threaded holes along the optics rail. If too loose or too tight they can cause issues with the bolt. They run in a slot along the top of the bolt, to prevent it rotating.
Anticipating the barrel nut problem, designing a latch to retain it when tightened was among the handful of modifications I thought about before even ordering the thing. It wasn't as high a priority as increasing the buffer weight with a tungsten rod, or replacing the buffer spring with a flatwire AR spring, or cleaning up the finish on any number of parts. Certainly not as important to me as modifying the trigger/sear such that pull weight dropped by about 40% and smoothed out completely, getting rid of the grittiness/unevenness many users talked about online. And there were other things I wanted to fix besides. But I could see that the barrel nut would work loose, and didn't want to have to think about that during a shooting session, so within a couple of months of getting the thing and having worked out all the other stuff, I made a latch. Here's what it looks like in action, without the barrel and nut in place:
Seeing that the tiny ball bearing on a weak spring, held in place by a tiny grub screw, was insufficient to do any real retention - all it did was make little ratcheting sounds while screwing in or unscrewing the nut, with marginal force - my first effort had been to just thread the hole and use a bolt to tighten directly into the nut, marking the spot where it got tight through the hole, then drilling, then using a flat ended milling bit to level the bottom of the hole at about 3mm below thread peak. This is just outside the taper which presses back on the tapered flange on the barrel, so don't want to go too deep and come right through and contact the barrel. I found this worked, but was annoyed with having to get a tool to loosen the bolt, so added a lever to the bolt to make it toolless.
That worked fine, but looked ugly. so I thought about it some more, then came up with a sprung plunger. Made a little knob out of micarta, then made a little rod with a small threaded end after a narrow shank, with a wider tip at the barrel nut end. A spring with decent resistance was put on the narrowed shaft so it would fit through a large grub screw. I drilled the receiver then tapped for a 1/4"-20tpi grub screw with a hole drilled through for the shaft, turning a flat end on the grub screw for the spring end.
Assembled by threading in the grub screw with epoxy (JB Weld steel-filled) on the threads, inserting the rod from inside the receiver with the spring in place, pressing upwards to compress the spring then threading on the knob with more epoxy on those smaller threads at the end of the rod which I deliberately made sloppy such that finger tightening would work, the epoxy securing it well once cured.
The result is something outwardly very simple, just a little knob which doesn't get in the way of my scope. Pulling it up about 3mm frees the barrel nut to turn. I hold it up for 2 or 3 turns until clear of the rod when taking out the nut. Marking the position for the hole was easy enough - just screw in the nut as firmly as I could, then starting the hole through the factory hole in the receiver. Any number of designs might be used, but this seemed the most fool-proof. The barrel nut simply can't come loose until I want it to.
