Building a .308 AR Carbine that works: Carbine Compatibility and Engineering

In the .308 AR Rifle article, we talked about the numerous .308 AR standards. This time we are going to cover tuning .308 AR rifles with carbine length buffer tubes. Even though standards abound, the truth remains; the easiest way to enhance reliability and longevity is to get the right buffer and spring combination in your gun.

The SR-25 carbine was built in the 1990’s when Combat Applications Group snipers asked for a 16 barrel and collapsible stock on their SR-25s for close in work requiring more agility.  Knight’s Armament built the guns and started a very good thing.

When Eugene Stoner (blessed be his name) originally designed the AR-10, it was purpose built for military 7.62mm M-80 ball ammo and a 20.8 inch barrel. The buffer system was designed for that combination. The AR-10 buffer was 5 3/16″ (5.188″) long and weighed 5.4 ounces. The use of a carbine buffer tube wasn’t considered. The three basic patterns of .308 ARs: SR-25, Armalite and DPMS all have carbine variants. There is no standard, but we now have a lot of options to come up with the right match. has a very helpful post :

LR-308 308AR AR10 AR15 Compatibility

The .308 AR receiver and bolt are longer than an AR-15 to accommodate the longer .308 cartridge. The careful balance of length of the carrier, the amount of travel required, the length of the buffer, and the compressed length of the buffer spring determines if and how well your rifle functions. I can recommend some solutions, but with all these variables, some experimentation is required. In fact, you might need more than one buffer and spring combination if you plan to change uppers or ammo.

Because the amount of bolt travel required for the .308 AR is longer, you cannot use an AR-15 carbine buffer with an AR-15 carbine buffer tube on a 308 rifle. You must have a specifically designed spring and extra short buffer.

The Tridentis 4.68  ounce .308 Heavy buffer is designed  308 Carbines.  With a length of 2.5 inches, the Heavy 308 will allow you to use any AR15 collapsing stock (Magpul/LMT/VLTOR/Ace/etc) on your AR10, SR25, LMT, POF or DPMS-pattern .308 rifle. 308 Heavy buffers are shorter and heavier than standard carbine buffers. They are constructed of 303 Stainless Steel with a sliding Tungsten anti-bounce counterweight.


A good match here is the Sprinco “ORANGE Spring”. This is the ONLY specifically engineered EXTRA POWER Buffer Spring for .308 Carbine Platforms using standard 7″ Depth M4 extension tubes and .308 short 2.5″ buffers. Some people clip coils from longer buffer springs to yield the proper spring loads for this application, but that is fraught with peril. I recommend Sprinco, particularly for heavily gassed .308 carbine platforms.

Which buffer and spring should you use in your rifle? That depends on your buffer tube, gas system length, the gas port size, ammunition, and if a suppressor will be utilized. I can recommend some solutions, but with all these variables, some experimentation is required. In fact, you might need more than one buffer and spring combination if you plan to change uppers or ammo.

I have seen many ARs of all calibers falter because of ammunition.  I recommend the use of high quality factory ammunition like Armscor .308.  You might get a deal on surplus ammo, but don’t blame your gun if it doesn’t cycle.  You can tune a gun for weak ammo, but you better re-tune it if you shoot the good stuff.

ArmaLite/VLTOR extended length tube rifles with a deeper ( 7-3/4″) .308 Carbine buffer tube uses AR-15 carbine size buffers. The Xtra-Heavy buffer is the heaviest carbine buffer on the market. At 8.5 oz it weighs nearly three times as much as a standard OEM AR15 carbine buffer so it helps reduce felt recoil. The Sprinco “RED Spring” EXTRA Power Buffer Spring, works great with these .308 AR Carbines (Armalite, Noveske, LMT).

Finding the correct buffer and spring will result in a high functioning rifle with an extended service life. A lighter buffer to reduce lock time and increase cyclic rate while a heavier buffer will keep the rifle in battery slightly longer reducing bounce and transfer more inertia to the bolt providing reliable chambering when dirty. Adjusting buffer weights can accommodate different ammunition or reducing the recoil impulse. A heavier spring with a properly tuned gas system can often yield a faster, lighter impulse that feels almost like an AR-15, very easy to clear or load.

Experimentation will allow custom tuning of any given rifle. Original manufacturers .308 AR springs and buffers spring should offer good performance over a wide range of configurations, but if you have any reliability issues, why not check a different buffer weight or spring? This alone can yield truly optimized performance, particularly for rifles without an adjustable gas block.

In selecting a spring for your rifle, install a heavier spring if you feel the buffer head fully compress and bottom out while firing the weapon. If the bolt carrier will not lock back reliably on the last round fired, even when completely gassed (in the case of an adjustable gas block), select a lighter spring.

Springs and buffers are cheap and they can keep your rifles healthy and happy.

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