It’s not just buffer weight but rather the entire system.
In addition to the recoil impulse of the round, you need to consider:
- entire reciprocating mass, (bolt weight plus buffer weight);
- the spring weight;
- buffer length;
- the type of buffer and how it inhibits bolt bounce; and
- whether or not you are running an automatic bolt hold open device.
Your basic starting point in a 9mm AR-15 should be:
- a stock 9mm carbine spring;
- an A5 length (4”) buffer;
- a buffer using internal moving weight(s) that will prevent the bolt from bouncing off the breech face; and
- a minimum combined bolt and buffer weight of around 23-25 oz.
You can go lighter with the total weight to increase the cyclic rate. But in general lighter weight means higher perceived recoil and a higher cyclic rate.
If you have a bolt hold open device a higher cyclic rate and commensurately higher bolt velocity increases the hit it takes after the last round is fired. That’s particularly the case if it is also either under sprung (with the buffer and bolt bouncing off the back of the buffer tube), or over sprung with the spring driving the bolt and buffer forward faster.
You can also go heavier to decrease the cyclic rate, but at some point the reciprocating mass starts to make it harder to control and bring back on target.