Exploding bolts (Dragon #348) and Thunder Arrows (Dragon #279) don't specify how many bolts/arrows come in a pack. Can I assume that they come in the regular 10/20-pack that bolts/arrows come in? Because it seems a bit illogical for a single arrow to weigh 6 pounds.
The table in Dragon #279 says specifically for Thunder arrows that the weight is given for 20 arrows. Thunderstones on the other hand cost 30 g.p. each. Thus the price indicated in the list (35 g.p.) probably indicates the price of a single thunder arrow.
A thunderstone nomally weighs 1 lb. - seems the elves must have found a way to create much lighter thunderstones for their arrows.