After some thinking I decided to build my own 9mm ar15, after the whole oly arms debacle (still waiting for my $...).

Anyways, I have ordered most of the parts I need.  I am going the RRA style route.
On RRA's site they say that a 9mm buffer is needed, so:
1) What does the buffer do?
2) Do I really need it?

ty

Depends.

Most people that have the 9mm setup have a collapsing stock.  In that case you need a 9mm buffer.  Take out carbine buffer put in 9mm buffer.  The 9mm buffer is heavier than the carbine buffer.  Since it is a blowback system more energy is forced back vs AR15 which is a gas system.  Even w/ a 9mm buffer the recoil is more noticeable in the 9mm carbine than in a rifle mode.

If you have an A2 stock then you can probally live with it.  Some people change it.

Quoted: Depends. Most people that have the 9mm setup have a collapsing stock.  In that case you need a "9mm" buffer.  Take out std carbine buffer (@2.9oz) put in 9mm buffer.  The 9mm buffer is heavier than the carbine buffer.  Since it is a blowback system more energy is forced back vs AR15 which is a gas system.  Even w/ a 9mm buffer the recoil is more noticeable in the 9mm carbine than in a rifle mode. If you have an A2 stock then you can probably live with it.  Some people change it.

The  ""9MM""  buffer is a CAR  style buffer that weighs very slight bit more than  the std A2 rifle buffer (5.6oz vs 5.1oz).....

I prefer the std rifle stock, but due to my running a suppressor, I took and gutted my buffer body and installed a machined brass dowel to fit inside,  bringing the weight to 7.0oz, this provides a delay in the opening, and reduce the amount of debris I get from the port.