Re: MK 12 Build
Like John said...IT DEPENDS!
With a rifle-length/A2 receiver extension, you'll need a rifle-length buffer.
With a carbine (or entry) receiver extension, you will need a carbine buffer.
The weight of the buffer is not always determined just by the stock/receiver extension you are using, but rather the configuration of the rifle itself, the ammo you are using, whether you will be shooting it suppressed all or part of the time, etc., etc.
If you can tell us a little more about your particular build and your intended purposes, etc., it would be easier to give you more detailed/specific advice.