I am doing my first build from scratch AR.  I have a Rock River 18" Varmint upper to start the project.  I know that different barrel lengths have different gas block pressures and different dwell times that are dictated by port to muzzle lengths.  I also know that there are different buffer weights and buffer springs that go with a particular barrel length / gas tube length.  Where can I find information on the proper buffer and spring to use with a particular barrel set up?  Thanks.

Excellent question Clay.  I am building a mid lenght carbine and I have no idea of what weight of buffer and springs would be ideal.  I will be using a A2 stock. Thanks!

If using an A2 stock then you'll need a rifle buffer & spring. If using a collapsible stock then you just have to play around with different carbine buffers to find the one that works best with your gun.

I will be using a plain vanilla A2 stock, so I guess the rifle buffer / spring is appropriate.  Thanks MUCH for the info.  Since I reload, I suppose the powder charge can be varied to get optimum performance.  My RRA carbine seems to be overgassed with NATO rounds.  The gun functions reliably with 55 grainers at 2500 fps.
Clay

Thanks Pig!  Clay, have you tired Varget?  27grns works great for me.  I also like 748.

24 grains of 748 with a 55 grain bullet gives me just over 2500 fps.  This is not the most accurate load for my rifle, but it is very mild.  I use it for matches and have had not problems with it.