Now that you figured out what the buffer is you can start having some fun with buffer weights. There is a standard carbine weight buffer and then there are heavier buffers that are supposed to reduce and smooth out the felt recoil and slow the cycling of the bolt. The heavier buffers are designated H, H2 and H3, with each being progressively heavier. The Spikes buffer mentioned by Christcorp uses powdered tungsten rather than metal weights. It weighs in between the H and H2 buffers, but gets rid of the rattling sound that buffers with weights can make.
I installed a Spikes buffer today and shot my S&W M&P15 Sport. The difference in felt recoil was somewhat noticeable, but did not seem that great. However, at the end of my range session my shoulder wasn't sore at all. With the standard buffer I would get some soreness in my shoulder after 50 or so rounds, so I guess the heavier buffers do make a difference.