The standard buffer is for full sized A2 stocks. The H, H2 & H3 buffer are shorter for use in CAR stocks - don't run them in a full size stock. (ask me how I know)
The difference between the CAR buffers is weight with the H3 being the heaviest.
This was a huge deal with my M16. I had the M4 stock and I tried all of the Buffers H-XH and NONE of them worked. I ended up giving up with the M4 stock and went with a full size Rock River operator stock because the only buffer I could get to run was the A2 buffer..Besides having full auto the only other reason you would need a different size buffer is if you are shooting SBR's or Suppressed weapons. Shoot both SBR and Suppressed in Full auto and it really gets tricky getting the right buffer. Too much weight it will short cycle not enough weight you get what I had BOLT BOUNCE...
Lol...Frost...story of my life. I was told by a friend in the army that every gun is different and sometimes not all guns work with the same buffer. They have a lot more funds to trouble shoot till they get it running. I just decided to quit before i ran out of money. I hate a fixed stock but it has grown on me cause it beats the alternative. The m4 stock was driving me to drink and cuss more than I already do.