Sacks are like boxes of bullets or packs of seeds, but they 2x2. They can't fit into the player's inventory model (the 4 1x2 columns) so there's no option to unpack them.

Also any weapon mod that doesn't fit will not give the upgrade option. Eg, if you have a mod that is 2x1 (wide) it won't give the upgrade option when r-clicking the weapon unless you rotate it to be 1x2 (tall).