Didn't know where to post this so I thought I'd try here. Just before I left the range this past weekend, I let the guy next to me shoot my just put together upper a few rds. As I stood  behind him, just to the left, I saw a flash just behind the delta ring and the upper receiver as he fired off a rd. He shot about three more without another incident. I packed up and took it home.
The upper was put together by someone who has done a couple of others for me and who was pretty well known on this board. I'm afraid to shoot it again and can't get to this person for a couple of weeks. It did have to go back just after I got it to correct and windage problem.  I was testing it the second time when I saw this.

What could be the cause of this?

What ammo was being used? There are some out there (S&B comes to mind) that will give you zero flash on one shot, and light up the room on the next.

PMP, Hot shot ss109, ultramax 68gr, old M193. The flash BEHIND the delta ring was with the old M193.

Where were you standing in relation to the gun/shooter?

Behind him just to the left. The flash came from in between the DELTA RING AND THE UPPER RECEIVER.

Maybe from the gas tube?

Quoted: Maybe from the gas tube?

that would be my best guess.  although, this would be a lot of flame to travel down the gas tube, off the carrier key and back out the space around the gas tube.  

it might be easier to tell where it is comming from if you watched someone fire it at night.

If there was a lot of soot left over from your previous loads, the gas jet from the relatively hot XM193 could have caused the particles to go incandescent.

I wouldn't worry, though, and would half-expect it to happen. The gas tube-gas key connection isn't anything near a perfect seal.

Okay, so while it isn't normal, it does happen and could happen again. Any danger to this and has anybody else experienced this?

Did you save the brass from the batch of ammo that caused the FLASH? If so, examine it for signs of high presuure, such as, falltened or cratered primers, extractor or ejector marks of the case head, if you have access to a micrometer measure the case head diameter at the web. The case head diameter of a fired acse should be no more than 0.0015 greater than that of an unfired case. I think you may have a bad lot of ammo, but, it is also possible that you may be close to having a firing out of battery due to excessive wear on some of the gun components. Be careful. Charles the Gunsmith.

I've seen it happen a few times on hot, dirty rifles that had just been liberally hosed with aerosol BreakFree to keep running. Flashover of the lube?  

Quoted: Did you save the brass from the batch of ammo that caused the FLASH? If so, examine it for signs of high presuure, such as, falltened or cratered primers, extractor or ejector marks of the case head, if you have access to a micrometer measure the case head diameter at the web. The case head diameter of a fired acse should be no more than 0.0015 greater than that of an unfired case. I think you may have a bad lot of ammo, but, it is also possible that you may be close to having a firing out of battery due to excessive wear on some of the gun components. Be careful. Charles the Gunsmith.

I did not save the brass, wish I would have. Everything is new except the bbl. which has approx. 300 rds. thru it. The upper rec. is DPMS and new, the bolt group is new (CMT), and the gas tube is new also. I think it could've been the ammo which was a bit dated. I'll try again with newer ammo. Thanks for all the help!

How is the fit between the upper and lower?  If you hold it up to the light can you see between them? How dark was it outside ? Sometimes in near or total darkness since the speed of light is faster than even our synapses can fire info to our brain. You may have registered the flash after it occurred and the image was pixilliating in your mind - just a thought
If it was the right side you were on I'd say you just saw some unburned powder getting flashed as the bolt cycled, but on the left side and behind the delta ring hmmm.  A hairline fracture ? You can rule out that the barrel was turned cross thread onto the upper because the first round would have went way over there and you would have been embarrassed because you just killed the shooter in stall #2 ...  
Remember in 3000 Miles To Graceland ... Costner was firing a full auto 9mm stubby they showed it cycling in slow mo the ejection port( I know its the right side)  looked the open door to the boiler in the grade school where I went, in other words it was on fire like the gates of hell. I am sure it was blanks he was firing and they were probably made very hot to do just what they did for Cinematic purposes , so Charles the Gunsmith may know his craft. I would heed his caution
take to the doctor ...IMHO

I just put a bright light on the gas tube where it goes into the upper receiver and saw some powder burns and it was on the delta ring as well.  Not much, but, it was there. Could it be a hairline fracture or something else? How can gas vent there from normal cycling?

So it was dark huh? You definitely saw an exray image? A hairline fracture could be possible but it would need to be disassembled to determine that. BM magnetic particle inspects their bolts for fractures so it could be quite possible that it could be a molding problem and that some degree of inspection would have been required to locate a crack in that particular upper. I doubt any reputable company shipped it out that way but you never know. If this is your only I'd take her to the doctor or get a new upper ...to be safe - you only got two eyes.