In Gunsmithing: The AR-15, Patrick Sweeney shares tricks and information gathered over 25 years of shooting and wrenching on the AR-15. The following tips are from the chapter on buffers and recoil springs:
The AR-15/M-16 rifle, as do all self-loading rifles, needs a recoil spring to cycle the action. Due to the lightweight design of the AR, the spring also requires a buffer, or extra weight. The earliest buffers were simply plastic tubes that rode inside the tube and in front of the buffer spring and gave the spring a bearing surface against which to work on the carrier and bolt. The carrier and bolt, blown off of the gas tube by the gases, cycle back into the buffer tube inside the stock, compressing the buffer spring. Once the energy of that action has been absorbed by the springs’ compression, the spring then uses the stored energy to push the buffer, carrier and bolt back forward.
Tip #1: Removing the buffer and spring
The buffer and spring ride inside a tube screwed to the rear of the lower receiver. The rifle tube can be recognized by its greater length and the lack of a rib. The carbine is much shorter (except for the “Post-ban” stocks) and has a rib on it to guide the sliding stock. Handgun-specific tubes lack both the “spine” for the telescoping stock slider, and the threaded hole in the rear of the tube for the fixed stock.
To remove the buffer and spring, push the rear takedown pin across and hinge-open the action. At the rear inside of the lower is a small plunger at the face of the buffer. Depress the plunger and the buffer spring will push the buffer out of the tube. Once the head of the buffer clears the plunger, grab the buffer and pull it and the spring out of the tube. It may take some wiggling to get it past the hammer (which should be cocked) but it does come out.
Tip #2: Measure your springs
The springs for the rifle and the carbine are the same diameter, and made of the same steel alloy. The rifle springs have 41 to 43 coils, and the carbine springs have 37 to 39 coils. The brand-new length of the springs is approximately 12.75″ for the rifle and 10.5″ for the carbine springs. If you’ve bought a used rifle or carbine, and find that a spring has been shortened by cutting off coils, it should be replaced. (Any rifle that has a shortened spring in it also needs a complete inspection, as the spring was probably not cut without “reason.” You’ll need to find what the problem was that led a previous owner/armorer/gunsmith/hack to cut the spring.)
Springs shorten with use. When a spring has gotten much shorter than the starting length, replace it. As long as a rifle spring is more than 12 inches long, and a carbine spring more than 10 inches long, they are still serviceable. When they have shortened to those lengths or shorter, replace them.
Tip #3: Count the coils
Rifle and carbine springs cannot be interchanged. While the lengths and number of coils would seem close enough that in an emergency you could get by, resist the temptation. “Emergencies” tend to linger on, as the equipment is “obviously” doing fine. It is hard on the rifle or carbine to be “getting by” with the wrong spring.
The longest spring I’ve ever observed was 50 coils long, The rifle it was in was regularly short-stroking and failing to eject. (Big surprise there, eh?) Count the coils!
Bonus Tip: Select a proper buffer
Selecting the rifle buffer is simple: there is only one. Except for the very earliest buffers, there is just the one rifle buffer: six inches in length.
For the carbines, there are a host of buffers, and one in particular is to be avoided. The shorter carbine buffer lacks the second spring shoulder found on the rifle buffer. Some of the carbine buffers can be found made of a plastic moulding instead of machined aluminum. These will be filled with lead shot. While they weight much the same as the proper aluminum carbine buffer, they are a cause of unreliable function. A carbine containing this buffer should have the wretched plastic abomination exchanged for a proper buffer.
The proper ones are made of turned aluminum, 3-1/4 inches long, with a nail-like head and a plastic tip. If you shake the buffer you can hear and feel the steel weights inside clacking back and forth. Those are the dead-blow weights. If a carbine works properly with a regular buffer, then leave it alone. If, however, you experience occasional malfunctions of unknown origins such as failure to extract, especially if you find the extractor has bent or broken the rim of the cartridge, replace the buffer with one of the heavier ones Colt developed. If your wallet can take the strain, the buffers on all your carbines should be replaced with H or H2 buffers. Even otherwise reliable carbines have a slight decrease in felt recoil when a regular buffer is replaced with an H or H2 buffer. In extreme cases, a carbine may need the H3 buffer.
Do not, under any circumstances, exchange buffers between rifles and carbines. The rifle buffer is too long, and firing a carbine with a rifle buffer in it will cause the buffer tail to strike the end of the tube at higher than designed velocities.If you are lucky, the result will be that the carrier key screws shear off. If the screws do not shear off in time, the lower receiver will crack at the buffer tube threads. The only repair for that particular mistake is a new receiver. Install the proper buffer, and avoid the hassles of a new lower to correct the mistake.