FAQ

AR-15 Drop-in Quad Rail Handguards

Will this drop-in quad rail fit on my “_____” AR-15? How do I know which size drop-in quad rail to buy for my rifle?

The AR-15 platform is standard across manufacturers and models including the:

  • Smith & Wesson M&P Sport
  • Colt LE6920 
  • Ruger SR-556
  • Bushmaster Carbon-15
  • Bushmaster Patrolman
  • DPMS Panther Oracle

For drop-in quad rails, however, you will need to confirm the length of the handguard before purchasing. Drop-in quad rail handguards install in place of the stock plastic handguard between the delta ring (located on the front of the lower receiver) and the front sight post/gas block. The size of quad rail you need will depend on the distance between these two points, or, in other words, the length of your gas tube system. The distance between the front of the delta ring and the rear of the front sight post should correspond with the following lengths: 6.5” for carbine length quad rails 8.5” for mid length quad rails 11.5” for rifle/full length quad rails. For more information, please view our guide on quad rail lengths:

AR-15 Quad rail buyer's guide

The carbine length quad rails are slightly longer than 6.5” in total length because the tapered end of the quad rail tucks inside the delta ring.  The exact length of the quad rail may vary slightly depending on the form factor of the model or on where you take the measurements.  But a quad rail advertised for carbine length AR-15s will fit any standard carbine length AR-15 guaranteed.

All of our drop-in quad rail handguards are designed for use with the standard circular front end cap that come with current M4/AR-15s. A number of older model mid or rifle length M4/AR-15’s have triangular style front end caps. In short, our mid-length drop-in quad rail handguards will work with either the circular or triangular front end cap. Our rifle length drop-in quad rail handguards will work with the circular front end cap only. It will not work with the triangular front end cap.

Please refer to our step-by-step installation guide here: AR-15 2 Piece Drop-In Quad Rail Installation Guide.

If you are having trouble getting the drop-in quad rail to fit, please double check to confirm that you are installing it the correct way. Some quick tips for installing your drop-in quad rail handguard:

- The tapered end (the side with the curved lip) should be directed toward the lower receiver.
- The tapered part of the quad rail should tuck inside the delta ring, compressing the delta ring slightly.
- On the front end, the two halves of the quad rail should come together around the circular end cap.
- The two halves of the quad rail will not screw all the way together. Tighten the screws until the top and the bottom pieces are flush against the circular end cap.



AR-15 Free Float Quad Rail Handguards

For a quick primer on the difference between a drop-in and a free float quad rail handguard, please see look at the following guide: AR-15 Quad Rail Handguard Buyer's Guide.

Our AR-15 quad rails do not fit the AR-10/308 style rifles.  We do carry quad rails and handguards for DPMS LR-308 style rifles.  For more information, please view our AR-10/LR-308 compatibility guide:

https://monstrumtactical.com/blogs/guides/80570499-selecting-a-handguard-for-your-ar-10-ar-308-lr-308

No. We do not recommend our quad rail handguards for use with the Adams Arms gas piston system. For a list of quad rails that do and do not work with the Adams Arms gas piston system, please view the following FAQ on the Adams Arms website: http://www.adamsarms.net/faq#qa1

Certain Bushmaster Carbon 15 rifles use a different diameter barrel nut thread size, standard mil-spec barrel nuts are not compatible with these rifles. Monstrum Tactical free floating rails will not fit these rifles (serial number E18000 or above) unless a Bushmaster barrel nut is used.

The inside diameter is 1.75” at the narrowest point.

Installation of a free float quad rail is a more involved process than installing a drop-in quad rail.  We recommend consultation with an experienced gunsmith prior to installation. Installation will require a vise, an upper receiver vise block, an AR-15 Armorer’s Tool, and a punch and hammer set. We do not yet have our own proper free float installation guide, but the step-by-step process for installing a free float quad rail handguard involves:

- Removal of flash suppressor
- Removal of front sight post/gas block
- Removal of delta ring
- Installation of the barrel nut. Please note that when screwing in the barrel nut, we recommend approximately 35 pounds of torque.  The barrel nut is made of aluminum and over-torqueing may result in damage to the barrel nut.
- If you are installing a low profile gas block, the installation of the low profile gas block
- Screw in the jam nut, Screw in the free float quad rail. Secure with the set screw.
- Replace the front sight post/gas block, if necessary
- Replace the flash suppressor


For a good tutorial on a free float quad rail installation, we recommend viewing NicTaylor00’s outstanding free float installation videos on Youtube:
Part 1: https://www.youtube.com/watch?v=wk0e9LA1wqM
Part 2: https://www.youtube.com/watch?v=rxylroMyQ2A

Free float quad rail handguards can be installed to extend beyond the gas block. This requires removal of the front sight post/gas block and installation of a low profile gas block. A low profile gas block is a gas block that is built low enough to fit on the inside of the free float quad rail handguard.  We do not carry low profile gas blocks, but they can be found online for less than $25. The step-by-step process for installing a free float quad rail handguard that extends beyond the gas block is

- Removal of flash suppressor
- Removal of front sight post/gas block
- Removal of delta ring
- Installation of the low profile gas block
- Installation of the barrel nut. Please note that when screwing in the barrel nut, we recommend approximately 35 pounds of torque.  The barrel nut is made of aluminum and over-torqueing may result in damage to the barrel nut.
- Screw in the jam nut, Screw in the free float quad rail. Secure with the set screw.
- Replace the flash suppressor

For a good tutorial on a free float quad rail installation, we recommend viewing NicTaylor00’s outstanding free float installation videos on Youtube:
Part 1: https://www.youtube.com/watch?v=wk0e9LA1wqM
Part 2: https://www.youtube.com/watch?v=rxylroMyQ2A



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