Long rails are technically heavier than an exact same version in a shorter length...but it can be negligible depending on the rail.
My DD Lite 12 FSPM rail with its mounting hardware is only about 2 oz heavier than a mid-length plastic MOE which would retain the stock barrel nut and end cap. ~2 oz. is a small price to pay (esp. with a LW 14.5" barrel) for all that room.
http://i111.photobucket.com/albums/n...s/DSCN1667.jpg
An 11" TRX extreme probably weighs less than a 9" MOE hand guard. So the additional length for comfort comes at very little weight expense with today's lightest rails.

