My FF tubes are a few ounces heavier than a standard set of same-length handguards and associated hardware would be.
But... most of them aren't exactly super-thin or aggressively vented.
I can tell you that my DPMS vented rifle length FF tube (15 oz) is about 3 ounces heavier than comparable standard handguards.
But, my "Todd Jarret" (
) YHM extended rifle tube adds another 7 ounces. It is almost double the weight of a standard set of rifle length handguards and hardware. (But, 'standard' handguards and hardware vary by manufacturer, as well. So, there's no 'set' weight.)