I have a RRA Entry Tactical LAR15 and wanted to put a red dot sight on it, but still be able to see my iron sights without having to put a special skeletonized see-through riser on the pseudo (very low profile) tactical ‘carry handle’, to avoid increasing the overall height of the rifle.  My solution was to mount the red dot sight on the quad rail as far back (close to me) as possible instead of on the tactical carry handle.  This allows me to look through my iron sights and the red dot at the same time, as it sits a bit lower on the quad rail than the iron sight on the tactical carry handle and aligns perfectly.  When the red dot optic is turned off, I just see my iron sights as normal (through a clear glass window), but when the sight is turned on, there is a bright red (or green) dot directly on the tip of my front site, as if it illuminates the front sight at the exact spot I need it too, kind of like a bright night sight that works during the day or a fiber optic front sight.

Seems to work great, but was wondering if there was any drawback to mounting the red dot on the quad rail as opposed to the flat top or the tactical carry handle like I usually see on most other setups.  

Thanks.  

In general, handguards arent as rigid to the barrel as an upper receiver, so they arent ideal.   That said, if your seems secure its probably fine for the type of shooting and accuracy most people expect from a red dot.   You could always sell the tactical carry handle and get a smaller rear sight.  Those seem intended to cowitness a red dot, but it sounds like your dot sits too high when mounted on the carry handle, so that thing isnt really the best setup for your gun.

I thought this video from Magpul was pretty neat- it shows starting at 1:17 how much movement there is between the upper receiver and the quad rail, even on an HK.  It's probably not enough to be an issue for a red dot, but it does suggest that you don't want to span the joint with your mount, red dot or scope.

Good info, guys.  Thanks to both of you.  That video was interesting, QuietRiot11 and I saw exactly what you were talking about.  The red dot sight is mounted just slightly in front of the gap on my rifle though and doesn’t span it, so I think I’ll be fine.