If you are at all familiar with the AR-15 quad rail market, you are probably familiar with the two main styles of quad rail handguard - drop-in and free float.  If you are not familiar, you can refer to our handy guide discussing the pros and cons of each.
Now allow me to throw another option into the mix - the Carbine Length AR-15 Drop-in Quad Rail with Forward Extension. Sometimes referred to as the Extended Carbine Length Drop-in Quad Rail or the Carbine Length Drop-in Quad Rail with A2 FSP Cutout, this rail is designed to combine many of the benefits of the drop-in and free float rails.
More rail space than the standard carbine length drop-in rail.
Unlike the standard drop-in quad rail which extends only to the rear of the A2 front sight post, the quad rail with forward extension extends several inches beyond the A2 front sight post.  It has a cutout on top for the front sight post and on the bottom for the sling and bayonet attachments.  Everything installs in place around the front sight without any gunsmithing involved.
This gives you a bit more flexibility in accessory placement or hand placement  You can attach your flashlights or bipods up front and still have room to handle your rifle at the near end of the rail.
Easier to install than an extended free float rail
The installation of an extended free float handguard on a carbine length AR-15 can be a hairy process.  It involves removal of the A2 front sight/gas block assembly and installation of a low profile gas block.  You will also need to remove the delta ring.  It requires some gunsmithing tools and a little bit of expertise in order to install it.  For most of us, this means taking it to your local gunsmith to have it installed for you.
The drop-in rail, on the other hand, simply drops into place around the front sight post and is held together with a few set screws.  Installation only takes about 5 minutes.  It requires absolutely no expertise and no special tooling whatsoever.
Retain use of the A2 front sight post
Another advantage of the extended drop-in quad rail over the free flaot is that it leaves the A2 front sight post intact, allowing you to use it as a front sight.  With a free float setup, you would need to purchase an install some type of rail mounted front sight if you wanted to use some type of iron sight system.
Heavier than the standard drop-in or free float setups
On the down side, the extended drop-in rail is obviously a bit heavier than the traditional drop-in rail.  It weighs about 18 ounces, which is about 50% heavier than the regular carbine length rail. It is likewise heavier than the free float setup, as you have the added weight of the rail in addition to the extra weight from the front sight post and the delta ring. 
Not ideal for long distance or competitive shooting
Also, for long distance or competitive shooting, you are not going to get the same level of accuracy as you would with a free float rail.  For most shooters, using their AR for home defense or tactical purposes, or for taking it to the shooting range every now and then, this impact is going to be negligible.  But it is something to keep in mind if you plan on using your rifle at longer distances or in competitions.
AR-15 Quad Rail Handguard with Forward Extension | Carbine Length | Drop-in | Black

All in all, the drop-in quad rail with forward extension offers an inexpensive and easy to install alternative for AR-15 owners who want more rail space without the added hassle of installing a free float handguard.

***Please note that this quad rail will not fit the Ruger AR-556 due to the rifle's irregular bayonet lug.***

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