AR Buffers and what they mean
Another interesting topic for the AR-phile is that of buffer weight. The buffer is a counterweight that works with the buffer spring to absorb the recoiling bolt assembly and then push it back into battery. It can be seen in this video in the area of the guns stock:
The buffer comes in different weight classifications and lengths. The buffer for a carbine is shorter than that of a full sized rifle. Buffers typically come in CARBINE/H/H2 and RIFLE sizes from lightest to heaviest.
Why should I care about buffer weights? Well... hotter loads and shorter barrels with shorter gas tubes result in greater operating pressures. This means that the bolt assembly is cycled faster. Sometimes it can cycle so fast that the bolt out-speeds the spring in the magazine that is trying to push the next bullet up into the chamber. This can be a big problem in automatic fire. The result is mis-feeds (or no feed at all) and more wear and tear on the guns parts.
A heaver buffer slows the system down and increases "lock time", which is the amount of time between when the round is fired and when the bolt begins to move rearward. This aids in extraction of the spent case and the heavier counterweight can reduce felt recoil. The following vid is for comparison:
One has to be careful that he/she doesn't go too heavy though, because then the bolt carrier may not move back far enough to strip the next round from the magazine. It may also not lock back on empty or cause other timing issues.