Yes it is a drop in two-piece rail, however despite no rotational play, I can very easily move the entire quad rail backwards compressing the delta ring spring in the process. Think of the same motion as pumping a shotgun where my quad rail is the fore end / pump grip. Is this normal to be able to move forward/backward this easily? I have probably put 120 rounds through it since installing the rail.

I have a drop in rail, and it has play in it. How far does it move? 1/2"?I doubt the rail is short, but it could be a possibility. I'll check mine when I get home from work.

How far fore and aft?  Does it do the same with standard handguards (although I assume that you don't have any or else you would have checked first)?  What rail?

Is your FSB pinned or clamp/screw? Did you take the FSB off to install the HG?

I'll have to measure movement when I get home. I would not say 1/2". It's not enough for the rail to pop off.. It's just awkward to be able to pull your whole rail backwards when you're holding the VFG. It's a Midwest Industries quad... Great rail. I'm wondering if it just loosened up from shooting.. Not sure what type of FSB I have.. Rail sits behind the hand guard cap. Same install process as default hand guards.

Only thing i can think of now that it was mentioned is your gas block.  If that loosened.  If your barrel nut loosened, and your barrel came out, I'd find that hard to believe. I think your being finicky. Drop ins are wobbley in all directions.  Get a free float.

That's what I was thinking. Maybe loosened just enough to make wiggle room but not enough to affect functionality

How hard is it to pull back the delta ring? All mine have had quite a bit of tention on them. The handguards "jiggle" but they don't slide back and forward.
Did you build it? Maybe your missing part of the spring. They are weld springs and while they don't usually fall apart anything is possible.

Here's my rail.. http://www.midwestindustriesinc.com/index.cfm?fuseaction=category.display&category_id=64

I've measured.. There is about a 1/8" gap between the portion of the rail that goes over the delta ring and where the receiver rail starts.. Both top and bottom rail pieces can move (independently of one another too) at least that far. Farther if that "bridge" wasn't there.. The delta ring is compressing as this happens. I did not build this rifle, a reputable company did and I can't imagine the barrel nut is loose. That would be a big bummer. I've had up to 4 different hand guards on it over the past few years maybe I wore the spring out? I can move the delta ring back with two fingers pretty easily, however the point at which the rail would actually fall out is harder to press to.

Can any of you (who have drop ins of any type) pull back on the hand guard and get it to move straight back toward the receiver?

Let me add that there is almost -zero- rotational movement in these hand guards unlike most drop ins

Those springs in the delta ring are comprised of 3 wafer springs that are tacked welded at 2 or 3 points...at least all the ones I've seen out are.  I have seen one of them come loose, causing the loose spring wafer to walk around the other 2 while they're still attached.  This causes a "too loose" situation with many handguards...not usually enough to let the handguard fall off.  Just one possibility.

I have a Mission First Tactical 7inch carbine drop-in quad rail.  It is rock solid with no play whatsoever.  My buddies AR which I recently shot, had a drop-in as well, and it had zero play as well.  We both also have vfg's on it too.  I'd check your delta ring spring.  You say you can compress the delta ring a good amount with 2 fingers?  I can't even move mine 1/4 inch without exerting force.  There's a reason they make a tool for it.  

I can't speak for the masses, but I don't think that's normal.

I would really have to push hard as **** to get it back far enough to actually have the rail fall out.. The first half of the way however is just too easy. Maybe I am being a little anal about it but this one isn't even close to it's first thousand rounds yet. Would I really want to do surgery on it is up to me I guess. As long as the barrel nut is fine. I would think with no rotational wobble its fine.

I just checked mine out.  I can move top and bottom independently from one another. I can grab them and they only slide down 1/16". But that's the gap on the tail of the rail that goes over the delta ring. It won't let me pop it off by sliding down.  I grab the delta ring and I need to push down very hard to pop them off. Either the section of rail that goes over the delta ring is short, or the gas block is not far back enough.

Sounds like you have the same rail as me, same result? I feel better about it. I just don't remember it happening before I put a couple hundred rounds through it post-install.

I don't remember these MI Gen 2 guards having that slot you're talking about maybe that's why they are more free to move forward/aft..

i was saying that your delta ring is fucked or your gas block is not back all the way or your rail is out of spec.  i can not pop these rails off with ease, i need to give the delta ring a solid strong pull to pop them off.  but they do wiggle.  its not a free float.  if you want rugged, get a free float.

my rail is a diamond head carbine length, came from stag as a complete upper.

Same here.. They wont come off unless I pull the delta ring all the way back, which is still difficult. It was just the movement forward/aft I was worried about.

I have a MI drop in on a BCM upper, it goes forward and aft a bit but it's not doing it without some effort. I really don't see doing it without trying.