I don't think it's sand; if it is, it's both extraordinarily fine and extremely compacted where it's in the barrel. On the other hand, when moistened, it seems to coagulate very rapidly - just like sand, but also like most other powders I've come across. Doesn't appear especially hydrophobic, in other words, but I can't tell if it's any more hygroscopic than sand. See attached picture, taken fresh in the cardboard packaging roll-cum-petri dish
