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The family of rifles generally referred to as "AR-10"s or "AR-308"s encompasses a variety of rifles from a variety of manufacturers.  Although similar in appearance and function, different rifles may have different upper receiver designs with parts that may or may not be interchangeable.  This situation makes shopping for a free float quad rail or handguard a bit complicated.
AR-10 quad rail
 
AR-10 style rifles have a few different commonly used barrel nut designs (the attachment point for a free float handguard), each of which is compatible only with a free float quad rail or handguard built specifically for that design.  The two most common standards are for the original AR-10 and for the DPMS LR-308.  Both of these standards are now used by a number of different manufacturers.
 
Armalite AR-10 Style Handguards
 
The original AR-10 was developed in the 1950’s by ArmaLite, who still owns the AR-10 trademark and sells the only rifle officially called the AR-10. ArmaLite AR-10 rifles use their own proprietary barrel nut standard which is incompatible with most AR-10/308 derivative rifle models on the market. The thread specifications for the AR-10 barrel nut are 1-7/16” and 18 TPI-3A. Besides ArmaLite, some popular rifle models that have licensed and use this barrel nut design include the Eagle Arms AR-10 the Noveske N6.  Handguards compatible with these rifles are offered by LaRue, PRI, Badger Ordinance and Daniel Defense among others.
 
DPMS LR-308 Style Handguards
 
Rifle maker DPMS would go on to develop their own AR-10 style rifle called the LR-308, with the same general design but with a number of parts incompatible across platforms including the barrel nut/handguard. The barrel nut standard based on the popular DPMS LR-308 has become the most widely adopted on the market.  The thread specifications for the LR-308 barrel nut are 1-7/16” and 16 TPI-
2A. Numerous manufacturers make rifles using the LR-308 barrel nut standard including the Remington R25 and the Bushmaster 308 ORC.
 
*Please note that while the barrel nut standard for the DPMS LR-308 rifles has remained the same, there are two different versions of DPMS upper receiver that differ in the height of the receiver rail.  Upper receivers manufactured before August 2009 have a high profile receiver rail, while upper receivers manufactured after August 2009 have a low profile receiver rail.  Handguards made for the low profile receiver will still fit the older receiver, but the top rail will not align squarely with the receiver top rail.
 
Monstrum Tactical carries low profile DPMS LR-308 pattern free float quad rails and keymod handguards in a variety of lengths at the lowest prices anywhere starting at just $69.95.  Each rail comes with a lifetime guarantee.  To view our complete line of DPMS LR-308 rails, please click here.
DPMS GII Handguards 
To confuse matters further, in 2014 DPMS released the DPMS GII 308 rifle.  The DPMS GII is likewise, a DPMS made .308 rifle with an AR style design.  However, the GII uses its own proprietary barrel nut standard.  GII barrel nuts are not interchangeable with LR-308 barrel nuts.
Other Non-Standard/Proprietary Handguards
 
Besides these standards, several other manufacturers of AR-10 style rifles use their own non-standard or proprietary style of barrel nut including Ruger SR-762 and the Daniel Defense DD5 V1.
 
AR-10/AR-308/LR-308 Free Float Rail/Handguard Compatibility List

Related Items

 
To confirm what type of handguard will fit on your AR-10/308 rifle please refer to our AR-10/308 hand guard compatibility list below.  The list is not comprehensive.  If you have any additons or updates, please send them our way.  Before purchasing a handguard, we highly recommending confirming compatibility with your rifle manufacturer.
 
LR-308 Pattern AR-10 Pattern Non-Standard

DPMS Oracle .308

DPMS .308 Classic

DPMS LR-308L

DPMS TAC20

DPMS Long Range Lite

DPMS NATO REPR

DPMS Lite Hunter

DPMS Recon

DPMS Sportical

DPMS Compact Hunter

DPMS LR-308B

DPMS LRT-SASS

DPMS AP4

DPMS MK12

DPMS 3G1

DPMS LR-308T

Smith & Wesson M&P 10 Series

Bushmaster .308 ORC (low profile)

Remington R25 (low profile)

Palmetto State Arms PSA PA10

Palmetto State Arms PSA PAC Tac10

ArmaLite AR-10 Rifles

Noveske N6

Eagle Arms AR-10

DPMS GII

Ruger SR-762 308 rifle

Rock River Arms LAR-8

LaRue Tactical OBR 7.62

LaRue Tactical PredatOBR 7.62 and

LaRue Tactical PredatAR 7.62

Daniel Defense DD5 V1

Sig Sauer 716 Patrol