Different buffer weights have been used over the years in AR-15 rifles and carbines. The heavier buffers were introduced to help functioning with shorter barrels where there was burst or full auto capability. The heavier buffer helps to slow down the cyclic rate and helps control bolt bounce.
Here is a list of the common buffers and weight.
Carbine 3 oz
H 3.8 oz
H2 4.6 oz
H3 5.4 oz
Rifle 5 oz
Spike's Tactical also sell
ST-T1 3 oz
ST-T2 4.1 oz
ST-T3 5.4 oz
Pistol calibers like 9 MM or 45 ACP usually have heavy buffers, around 8 oz or more. Pistol caliber AR-15's are generally blow back actions and require a heavy buffer.
Here is a list of the common buffers and weight.
Carbine 3 oz
H 3.8 oz
H2 4.6 oz
H3 5.4 oz
Rifle 5 oz
Spike's Tactical also sell
ST-T1 3 oz
ST-T2 4.1 oz
ST-T3 5.4 oz
Pistol calibers like 9 MM or 45 ACP usually have heavy buffers, around 8 oz or more. Pistol caliber AR-15's are generally blow back actions and require a heavy buffer.