A stock carbine buffer is basically a 2.9oz weight that serves to slow the bolt carrier group down and then help push it back to the locked position after it has cycled.
Various options are available and getting an aftermarket buffer can either improve the feel of your rifle while shooting, make no noticeable difference, or turn it into an unreliable weapon. It all depends on how your rifle is configured and, to a small extent, how you shoot.
Depending on who you talk to, some people say a lighter buffer/BCG makes for less recoil, some say a heavier buffer does the same. It is all very confusing, and nobody wants to spend money on a part that might make their rifle worse. So what one IS actually better?
Enidine Hydraulic Buffer left and Spikes H2 buffer right
The quick answer is that in most cases a heavier buffer like Spikes H2 (4oz) or H3(5.4oz) will make a pistol or carbine length gas system feel smother. The H2 usually helps with a midlength system, the H3 can cause it to short stroke, depending on the exact gas hole diameter and gas block itself. I would not advise any increased weight buffers in a rifle length gas system, though I have buddies who swear by the H2 for that.
Then there is the Enidine hydraulic buffer. It does the same thing as the H2. Some say it does a better job. And they may be right - it does feel good, but I still think it just adds more weight and again, while effective in mid to pistol gas systems, I don't advise it for rifle length.
JP Rifles "silent captured spring" is a spring/buffer combined in one
Another option if you have the money is the JP Rifles silent captured spring, which is a return spring and buffer combined into one part. I have not shot one yet, but initial reviews are excellent, though it is worth noting that JP does a couple midlength gas barrels, but the rest are all rifle length.
Finally, while not a buffer, you can add weight to the bolt carrier using something like the Tubbs Carrier Weight. It does the same thing, just for a little more money and a little more work. They imply it is different to a heavier buffer, but without testing one myself I am forced to rely on physics, which says whether the extra mass is on the carrier or in the buffer, it has the same effect.
The sad thing that has to be said though is that any new buffer in any rifle will effect the tuning of the gun and could either improve or worsen the rifle's feel and reliability depending on the gas system, gas block, BCG and ammo used. If you really want to tune your rifle, you can use any buffer. You just need an adjustable gas block and try and stick with similar ammo.
If you wonder about the "why" for any of the above, read on but we warned - high school physics gets a mention.
For the most part the standard buffer works well. But many shooters prefer to the feel of the increased buffer weight as it does seem to reduce recoil. The effect comes from slowing the bolt carrier down, which makes the recoil feel more like a shove and less like a punch and therefore easier to control. It is especially noticeable in a carbine/pistol length gas system. And it makes sense to get a heavier buffer for that reason. The force is coming back at you anyway, so why not make it into useful energy that can also work the bolt forward under more adverse conditions, like after you have fired a lot of rounds and carbon has built up? It also reduces the sharp impact of the buffer bottoming out in the buffer tube.
So to reduce felt/perceived recoil you can make your buffer heavier. But we are also told by some places to reduce recoil you can get lower weight (mass) bolt carrier group. Which makes no sense right? - If more weight reduces recoil then how can less weight also reduce recoil? JP says use a low mass carrier and Tubbs sells weights to make your carrier heavier.
Well, it makes sense that less weight moving around inside the rifle during shooting would mean less movement to control and less recoil, but that contradicts the fact heavy buffers help also.
The explanation lies in 2 words: Speed and Balance.
A heavier buffer does not, in fact reduce recoil. It just spreads it over a fraction of a second longer. You don't get any less energy in your shoulder, you just get it more smoothly which makes it feel like less. If you double the time it takes for any given amount energy to be exerted, it halves how strong it is felt.
To give an extreme analogy just think about driving. Adjusting your speed from 0-60 or 60-0 takes the same amount of energy. But taking 35 seconds to get to 60mph in a minivan feels like nothing (other than frustrating anyway). Doing it in 4.0 seconds in a tuned muscle car feels like more of a kick in the ass. Same with braking: Going from 60 to zero in 2 seconds is an awesome display of braking power, but if you do it in 0.2 seconds it is usually fatal.
So if you increase the buffer weight, you are slowing your rifle down. That does not affect everybody in any negative way, but it can affect some shooters by adding fractions of a second to their time on rapid drills. Or, if they are used to running their gun as fast as they can at semi-auto, it can cause them to get trigger lock by disrupting their timing - they start to pull the trigger again before the bolt is fully home. If you don't care about the gun running a tiny bit slower or an ounce of difference in overall weight, then it is no big deal.
Holy crap you are still reading this. Ok, I will go into some more:
By "balance" I mean the balance of components, or the tuning of your weapon.
The gas that cycles the weapon hits either the gas piston, or the carrier key with a certain amount of energy. How much depends on the gas hole diameter, the gas block, how far down the barrel the hole is and the ammo used. An adjustable gas system lets us change that energy some.
Newtons first law says that for every action, there is an equal and opposite reaction. There are 2 that we feel: The action of the round accelerating down the barrel, and the gas cycling the rifle. We can't change the reactive force of the round traveling down the barrel without changing its muzzle velocity - which generally we want as fast as possible. But we can change how much force the gas exerts on the carrier key/gas piston, if we use an adjustable gas block. Less force=less recoil.
We want it to be in tune with the weight of the buffer combined with the BCG and the force the buffer spring puts on those parts. Too much force from the gas and the recoil is sharp and parts are getting overworked. Too little and it wont make it back far enough strip a new round off the magazine (short stroke). Ideally, if you are increasing the buffer mass, you would want a stiffer return spring. But generally it is not needed.
So adding weight can slow your cyclic rate down, reducing weight can speed it up. Doing either can make the weapon easier to control if it is tuned, or harder to control if takes it further out of tune. Most carbines are far from tuned- they are generally overgassed. So the H2 or H3 or Enidine or Tubbs weight helps. But if you get a rifle length gas system you can crank the gas down a little, lighten the carrier and you have a low recoil gun that runs faster. There is less force working inside to overcome dirt or carbon build up, but you have a fast, smooth running gun.