Compatibility Guide for AR-15 Drop-in Handguards

Compatibility Guide for AR-15 Drop-in Handguards

Posted by Monstrum on Apr 19th 2021

We commonly get questions regarding which AR-15's will fit our drop-in handguards. We have built a list below of some of the most common AR-15s sold in the market and which of our drop-ins they fit. If your AR is not listed on here, don’t worry, we have a short guide here as well on how you can tell if your rifle will be able to utilize a drop-in rail. We also made an installation tutorial for installing drop-in handguards.

Carbine Length
Carbine-length refers to the gas system length, the distance from the gas port to the chamber. Carbine-length gas system are 7.5" in length and carbine-length handguards are around 7". We offer two types of carbine-length drop-ins. One is a standard carbine-length drop-in and the other is an extended rail drop-in that also fits carbine-length AR-15s, images below. Here is a compatibility list of the most common carbine-length AR-15s in the market that will fit with our carbine-length drop-in quad rails:

Smith & Wesson:
M&P 15 SPORT II - with or without A2 Post

Ruger: (Only our standard drop-in will fit, not extended drop-in)
AR-556: 8500, 8515, 8502, 8511, 8523, 8580, 8503, 8504, 8507, 8517, 9519

Palmetto State Armory (PSA):
M4 Carbine-Length Battlelink Pistol
PA-15 M4 Carbine Classic Rifle/AR-15
Carbine Classic AR-15 Pistol
M4 Carbine-Length Freedom Rifle

Rock River Arms
CAR A4: LAR-15M, LAR-9M, LAR-15-LH

Aero Precision

M4: Carbine, Carbine OEM1, M4A1 SOCOM
XM177E2 Retro

FN Herstal
FN 15 Military Collector M4

Anderson Manufacturing
AM-15, AM15-BR

American Tactical
ATI AR15 OMNI HYBRID MAXX: 22LR, 5.56/.223

Mid Length
Mid-length gas systems on the other hand, are 9.5” in length and the handguards that accommodate them are 9” in length. One thing to look out for when it comes to mid-length AR-15s and that is the shape of the handguard cap. They come in two shapes, triangle or circle. The most popular handguard cap shape being triangle for most popular brands out there. Below if an image that illustrates what they look like, and lucky for you, we carry both.

Here is a compatibility list of the most common mid-length AR-15s in the market that will fit with our carbine-length drop-in quad rails:

Palmetto State Armory (PSA)
PA-15 Mid-Length Dissipater Rifle – Triangular handguard cap

Aero Precision
AERO AR-15M Mid-Length - Triangular handguard cap

Smith & Wesson
M&P 10 – Circular handguard cap

Springfield Armory
Saint – Triangular handguard cap

Rock River Arms
Mid-Length A4: LAR-15M, LAR-9M, LAR-15-LH – Triangular handguard cap

Identifying AR-15 Compatibility
There are two important items on stock ARs that are needed to fit drop-in rails: a delta ring and a handguard cap.

Figure 1: The delta ring is located between the handguard and the upper receiver.

Figure 2: The handguard cap is located right behind the gas block. Carbine handguard end caps are always a circle while mid-length handguards vary between triangle and circle.

Now that you know about the two main items needed on a stock AR for drop-ins to fit and their various lengths you can confidently identify which drop-ins will fit your needs.

AR-15 without a delta ring or handguard cap
If you have an AR-15 without a delta ring or a handguard cap, it means that you have a free float handguard. More manufacturers are building AR-15s with free float handguards as they are seen as more desirable by some, but it also comes with additional costs. Sometimes you end up with a mounting platform that you do not want. If this is your case, we also offer free floats in the main mounting platforms: PicatinnyKeymod, and M-LOK.

If you want to convert your stock AR-15 to have a free float handguard, we also have instructionschecklists, and all the tools and parts to make it easier for you.