Why would you want to put a full length quad rail on a carbine length AR rifle? Two words: Sight Radius. Sight radius is the distance between the front and rear sights. A longer sight radius aids accuracy because it magnifies (at your eye) error in your sight picture allowing for better error correction. A significant portion in accuracy loss of a carbine vs. a full length rifle is due to a loss of sight radius, not just the length of the barrel alone.
I decided to build my AR M4gery with a longer sight picture than a traditional M4. Since the traditional A-style front sight is part of the gas block assembly, the sight cannot be moved. So the traditional front sight had to be eliminated by installing a low profile gas block that does not have in integrated front sight. For an already built rifle, you could replace this assembly or cut the sight off of the gas block with a Dremel. Either way, this allows you to install a long quad rail that extends past the gas block. You can then mount a front sight at the end of the quad rail beyond the gas block. The sight radius of a traditional M4 is about 15 inches. Because of the long quad rail on my AR15M4, my rail mounted front sight is a full 19 inches from the rear sight. This means that I have achieved a 4in longer sight radius over a M4 with the traditional A-style front sight that would have been just forward of the gas block.
While I like the side profile look of the old A-style AR front sight, I really don’t like it when using it to actually aim the rifle. The open front sight does not really compliment the rear peep sight. What I mean is that the “dog ears” on either side of the front sight post open away from the post and do nothing to aid in sight alignment. However, the “ears” on the HK style front sight close around the front post forming a front aperture which can be aligned with the rear aperture (at the rear sight) which amounts to basically aligning circles. And the human brain is real good at aligning circles. So a HK style front sight compliments a rear peep sight very well and aids in fast sight alignment. I use an HK style front sight on my AR15M4 and I love it. I opted for folding BUIS (Back Up Iron Sights) in case I decide to add optics to my rifle. The BUIS (Front and rear) on my AR15M4 are made by Troy Industries as is the full length quad rail.
IN SUMMERY:
PROS:
1. Longer site radius means better aiming with iron sites = better accuracy.
2. Can choose a front site (like an HK style) that works better with the rear peep site than the standard AR front site.
3. Can choose to co-witness the front site with an optic or fold-down the front site when using an optic.
CONS:
So, what are the tradeoffs?
1. Even though the full length quad rail does not add allot of weight; what weight is added is on the far end of the rifle and when aimed the weight is effectively at the far end of a lever making the weapon feel front heavy. This is made even more apparent if you take advantage of the quad rail by adding accessories like lights and lasers.
2. If your primary means of aiming the weapon is going to be a optic of some kind, the benefit of a longer site radius is not so important because the iron sites will be seldom used.
