In conventional AR-series weapons, the buffer system consists of three main parts and a detent for retention purposes. There are certain exception cases, which I'll get out of the way first, which do not use them.
Exceptions to the Rule
There are certain weapon systems which utilize a proprietary buffer system, or do not require one. A short but incomplete list is:
ZM Weapons' LR-300 rifles. This is because they use a custom short-action bolt carrier design and do not use a conventional buffer assembly. A rule of thumb is that if you're looking at a side-folding rifle with a hinge right behind the action, it does not have a conventional buffer assembly. The Para Ordnance AR-15s were produced under license from ZM Weapons, from their introduction until Para exited the rifle market on 9/29/2011 and reassigned the manufacturing rights to this design to ZM.
Some 22LR uppers do not require the use of a buffer spring or buffer.
Most 50 BMG (or other large caliber) bolt action uppers don't have enough room for a buffer or spring and a bolt. Moreover, in the Bohica Arms example, the users are specifically warned NOT to leave the buffer spring in the weapon due to safety concerns.
Typical Buffer System
The typical buffer system in an AR-series weapon consists of the following parts:
Buffer Tube / Receiver Extension: This is a tube which screws onto the large threaded hole on the back of an AR-15 receiver, and provides the buffer and bolt carrier with somewhere to go as the action cycles. The most common internal tube lengths are rifle and carbine, although I think some pistol tubes are shorter.
Buffer: This is a weighted object, generally with an impact-resistant polymer tip on one side (turned down so that the spring slips over it) and a flat striking surface on the other. Whenever a round is fired, the bolt carrier is driven backwards and impacts the striking surface, pushing it to the rear of the buffer tube.
Buffer Detent: This is a spring-loaded detent which prevents the buffer from remaining in contact with the bolt carrier at all times. Although the gun will run without one, it's essential to the ready breakdown and reassembly of an AR-15.
Buffer Spring: This spring is held in the tube by the buffer, which in turn is held in by the detent. Buffer springs are available in different lengths and weights, for different applications.
Buffers come in a variety of shapes and sizes. I'm presently trying to determine what the actual official specifications are. Army TM 9-1005-319-23&P shows that a carbine buffer is shorter than a rifle buffer, and a carbine buffer is a simple "bullet shape" while a rifle-length buffer has another disc and rod atop the bullet. Unfortunately, the training manual does not call out expected weights for the buffers.
This was pulled from a post by madcratebuilder on this thread.
Here are the buffer weights in case you don't have them. These weights have a +/- of .1oz.
Carbine= 2.9 oz -> 3 steel weights
H buffer= 3.8 oz -> 1 tungsten weight
ST-T2= 4.3oz. -> tungsten powder
H2 buffer= 4.6 oz -> 2 tungsten weights
ST-T3= 5.4oz -> tungsten powder
H3 buffer= 5.6 oz -> 3 tungsten weights
Rifle buffer= 5.2oz
The ST-T3 is very close to the rifle weight.
Vltor A5 RE system. The Vltor buffers requires the Vltor A5 RE.
Vltor standard A5 buffer weighs 5.3 oz
Vltor H3 A5 buffer weighs 6.1oz
Vltor H4 A5 buffer weighs 6.8oz
About Buffer Springs
Per the ominously worded US Army TM 9-1005-319-23&P (which I haven't mirrored because it's plastered with export restrictions and I don't want to be responsible for worrying about what countries are reading this site), a rifle-length spring should be between 11.75" long and 13.5" long. The carbine length spring should be beteen 10.06" long and 11.25" long.
-- SeanNewton - 01 Sep 2012