I just installed a new dust cover on one of my 300BLK builds per the wifes request so it would be easier for her to identify the calibers.  After the PITA of doing everything involved with taking off the barrel, putting the new cover on, then reassembling, the damn plunger or bearing or whatever you call it that holds it closed fell out after just a few cycles (by hand).  While pretty pissed, I figured since I didn't recall ever assembling one of those, I'd try to view this as an opportunity to learn how to do it.  After a lot of looking with lots of different phrasing, I can't find a single video or article discussing how to replace/repair that part on a dust cover, nor can I find the parts just for that, it's all dust covers already assembled by themselves or in the kit with the hinge, spring & c-clip.  I thought, well I have a few extras laying around I'll just disassemble one & put the parts in the new one.  After looking real closely at them, it looks like the plunger is held in by some kind of roll pin, but it seems to only have one hole, like once it's driven in, that's it, unless there's some special tool to remove them.  Anyone have any experience building/repairing one or where to find the parts or some sort of guide to fixing it?  I really don't want to have to take the whole damn thing apart just to put it together again to replace the dust cover, again.

I would think that would be a defect in the upper receiver.  I'd try to return it.

I've had it for years & never had a problem with any part prior to this, so I'm inclined to think the problem was the dust cover, why would you think the whole upper is bad?

Pull c-clip of the dust cover shaft and remove shaft to rear of rifle? Throw away dust cover. Install this: https://gasblockgenie.com/easy-install-dust-cover/
If you like, add a sticker to identify caliber.

It’s a single use assembly from everything I’ve seen.  Once a roll pin is driven in, its not coming out.  Most likely whoever manufactured it forgot the roll pin or didn’t have the detent pushed far enough down when the roll pin was installed to allow it to be captured.  

Contact the manufacturer and have them replace it or pick up a new one.  Those are basically your choices.

The detent in his port door popped out.  How is that an upper problem?