Random Thoughts on AR Lower Receiver Extensions
by Greg Ritchie
[D.S. ARMS - AR-15/M16 MIL-SPEC BUFFER TUBE - available at Brownells]
AR receiver extensions? What are the differences? First, let's answer what they do and how they do it. The semi automatic rifle requires a recoil spring to operate. In the case of the AR15 this spring rides in a tube that is attached to the rear of the receiver. Because of the light weight of the AR15 bolt, it requires a buffer to help slow the rearward movement of the bolt. The compressed spring / buffer assembly then pushes the bolt forward into battery. The buffer is the mechanism used to time the rifle so it functions correctly. We will discuss how to time your AR15 later. First, let's look at the tunes, springs, and buffers.
There are three types of tubes. The rifle tube which is recognized by its greater length, lack of a rib to adjust a telescoping stock, and a threaded hole in the back of the tube used to attach the stock. The carbine tube is shorter and has a rib to adjust the telescoping stock. And finally the pistol tube, which is basically a carbine tube without the rib or threaded hole to attach the stock.
There are basically two different springs, the rifle spring, which should be 12 3/4" long with approximately 42 coils and the carbine spring, which should be 12 1/2" long and have approximately 38 coils. You should count your coils, anything much over or below the above stated coil count will cause you problems. Both springs are made of identically specific material with the same coil rate, the only difference being the length.
Finally we have the buffers. Buffers look sort of like an enlarged nail and have dead blow weights inside of them. There is only one rifle buffer. It has a second shoulder to capture the spring and has 5 steel weights and a steel spacer. The rifle buffer weighs 5 ounces. There are five different carbine type buffers. These are 3 1/4" inches and lack the second spring shoulder. The first is an economy type buffer. It's made of plastic and is lead filled. Stay away from this one, make sure any buffer you buy has at least an aluminum body. Next is the standard carbine buffer which has 3 steel weights and weighs 3 ounces. Next is the H buffer, which has 1 tungsten and 2 steel weights, this one weighs 3.8 ounces. The H2 has 2 tungsten and 1 steel weight and weighs 4.7 ounces. And Finally the H3 buffer, which has 3 tungsten weights and weighs 5.6 ounces.
Before someone cries foul and states there is this specialty spring and that specialty buffer, yes, you are correct. But for the sake of this discussion, let's stay with standard equipment.
Which buffer should you use? For a rifle items answer is obvious because there is only one. For a carbine, start with a magazine and 1 round and the standard carbine buffer. Fire the round. Did the last round bolt hold open engage. If yes, go to the H buffer. Again load a magazine with 1 round and fire it checking that the last round bolt hold open engages. Do this using the H2 and H3 buffer until the last round hold open does not engage. Go back to the next lightest buffer and your AR15 is properly timed for that particular round. Be aware that if you change anything, say you are timed for a 62 grain bullet and you drop to a 55 grain bullet, it is not unheard of to have to retime your carbine. The idea is to use the heaviest buffer possible to reduce wear and tear on your rifle. In my experience, most carbines function best with a H or H2 buffer.
One final note. Carbine and rifle buffers are not interchangeable. A rifle buffer in a carbine will bottom out and cause the carrier key bolts to shear and the receiver to crack.