Does your 308 rifle use a DPMS LR-308 pattern barrel nut and handguard?
AR/LR 308 style rifles use several different barrel nut and handguard standards, depending on the make and model. Before purchasing a handguard, please make sure you are purchasing the correct one. View our 308 compatibility guide for more details.
Why install a free float handguard?
One of the main benefits to free-floating a DPMS LR-308 is an increase in accuracy from reduced variability in the barrel harmonics due to sling tension. The traditional LR-308 configuration has the sling mount attachment at the base of the front sight gas block, so moderate changes in sling tension can change the point of impact downrange. A direct impingement LR-308 can never be truly “free-floated” due to the attachment of the gas block and gas tube, but the addition of a quad rail or other free float style handguard can help with accuracy and also provides a stable and convenient platform for attaching bipods, lights, grips and other accessories.
What do I need to install a free float quad rail handguard?
When planning your free-float project, take a close look at the type of gas system currently on your rifle. Most standard front sight gas blocks are held onto the barrel by either tapered pins or by clamps. Make sure you have all the right tools and parts on hand for disassembly and reassembly before starting work.
As always, before working on any firearm make sure it’s unloaded and safe to handle.
First, remove the original handguards using a handguard removal tool.
Separate the upper and lower receivers and remove the bolt carrier group and charging handle.
Clamp the upper receiver into a vise using a vise block and remove the flash hider and crush washer. The crush washer will not be reused.
Tap out the gas tube roll pin with a #2 roll pin punch and inspect the front sight gas block. If it has tapered pins, drive them out with a cup tip punch from the small side. If unsure, measure the pin diameter on each side first. If the pins are difficult to remove, apply penetrating oil, let sit overnight and try again. Sometimes applying light heat to the area also helps with removal.
If the gas block is a clamp-on type, use a hex key to loosen the set screws. Remove the original front sight gas block assembly from the barrel. If it sticks, gently tap on it with a soft-faced hammer until it slides off the barrel. Remove the handguard retainer and gas tube.
On a bench block, align the gas tube through-hole (with the large hole facing down) into the new gas block and use a roll pin holder to start the roll pin. Drive the roll pin flush with the gas block.
Remove the original barrel nut/delta ring assembly with the armorer’s wrench, then clean and inspect the upper receiver threads.
Remove the anti-rotation screw from the bottom of the new handguards and unscrew the new barrel nut, removing it from the handguards. Apply a thin layer of high quality lithium moly grease to the inside threads of the barrel nut, install it over the barrel so that the lock ring faces toward the upper receiver, then hand-tighten the barrel nut.
Set your torque wrench to 30 ft/lbs and tighten the barrel nut using your armorer’s wrench. Remove the torque wrench and loosen the barrel nut with the armorer’s wrench (never loosen with a torque wrench).
Re-torque to 30 ft/lbs, then remove the torque wrench and loosen again. Torque to 30 ft/lbs one more time and inspect the alignment between the barrel nut hole and the gas tube hole in the upper receiver. This three-time torque sequence allows the threads on the upper receiver and barrel nut to mate more completely.
Without exceeding 60 ft/lbs, tighten the barrel nut until the gas tube hole in the upper receiver and the hole in the barrel nut align perfectly. The holes should line up at about 35 foot-pounds of torque, and the gas tube should be centered in the barrel nut hole without touching the edges. Do not partially loosen the barrel nut to achieve alignment – if need be, loosen the nut completely and re-tighten to align correctly.
Apply blue threadlocker onto the gas block set screws and slide the gas block assembly onto the barrel, making sure the gas port in the barrel aligns with the hole in the gas block. Tighten the set screws on the gas block securely.
Next, loosen the lock ring and carefully thread the quad rail onto the barrel nut for a couple turns. Apply blue threadlocker onto the outside barrel nut threads and screw on the quad rail.
Adjust the lock ring and handguard until the top rail (marked “T”) is almost aligned with the rail on the upper receiver. Use a strap wrench and carefully tighten the handguard until snug. Be careful not to overtighten; if too much torque is applied, it can misalign the barrel nut and gas tube underneath.
You can check for proper gas tube installation using the bolt carrier at this time: insert the bolt carrier group (without the charging handle) into the upper receiver and slowly slide it forward, checking to make sure the carrier gas key doesn’t bind on the gas tube.
Take a reading with your level across the top of the upper receiver and compare it with the level of the quad rail. Adjust the quad rail rotation until it matches the level of the upper receiver.
Reinstall the muzzle device with a new crush washer and align it correctly (flash hiders generally with the vented ports facing upward; muzzle brakes generally with the ports facing sideways).
Then apply blue threadlocker onto the anti-rotation screw on the bottom of the handguard, install it, and tighten until snug.
Reinstall the charging handle and bolt carrier group, then reassemble the upper and lower receivers and you’re done!