Note on this photo... there is a half thread on the bottom of the tube that is ground away on the upper portion of the tube. Can you see it on each side?
This is correct.Many times a carbine tube is cut straight across the tube. Then, when it is threaded into the receiver to properly hold down the detent, as you can see in this photo, with the tube almost, but not quite touching the "tit" on the detent, then the threads protrude at the top. That prevents the upper from closing.
So some dufus unscrews the tube one full turn. Now it is back enough to allow the upper to close, but does not sufficiently hold the detent.
The correct thing to do is simply grind back that upper thread, easily done on a bench grinder and then dress off the rough spots with a wire brush.
Better quality carbine tubes come with that upper thread already trimmed back. And from the appearance in this photo, this was one that was properly shaped to begin with.
Get a new tube and fit it as I have just described.
In the next photo, this tube is not properly installed. It does not thread in enough to properly capture the detent. The person that installed this tube was afraid it would protrude too much at the top and bind with the upper receiver. He was correct. As it is, it may work for a short while, but soon a little wear on the edge of the tube will allow the detent to get loose.
FAIL!This next tube is not properly installed, either. It is threaded in enough to capture the detent, but the top thread of the tube protrudes past the upper receiver where it will bind and wear on the upper receiver.
FAIL!These two improperly installed tubes are not the fault of the DPMS receivers. They would be that way in any receiver.
Are those photos of Ace stocks? The Ace tube is cut with a longer lip on bottom.
What they are is the result of poor installation.