Good News Translation‘s paragraph break here is good.

Whoever throws a stone straight up throws it on his own head: Good News Translation rewords this as an imperative, saying “Throw a stone up in the air and it will come down on your head.” We could also translate “If you throw a stone … it will come down on your head.”

And a treacherous blow opens up wounds: The meaning of this line is not clear. New Jerusalem Bible‘s footnote offers a literal reading: “and a treacherous blow distributes wounds.” The image of a sword is presumably in view here, and a treacherous blow by a sword is one that is struck badly, that is dangerous to the swordsman, either because he wounds himself or leaves himself open to being wounded by an opponent. With the verb “distributes,” the sense may be that the treacherous blow is not well-directed, and can hurt people it is not intended to hurt, whether the swordsman himself or others. A sword is not like a modern weapon that kills indiscriminately; it is not supposed to distribute or divide wounds. It is supposed to wound a particular target. If it does not, the blow is treacherous. We could translate this line as “If a soldier is not careful, he may wound himself with his own sword.”

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Sirach. (UBS Helps for Translators). New York: UBS, 2008. For this and other handbooks for translators see here.