This is book 18 of the "Rick Brant" series.

This note contains spoilers for the book.

A Rick Brant science adventure! Publised as by John Blaine, as they all were.

Scotty and Rick are cruising the Chesapeake, going to visit Steve Ames at his summer home, when they encounter a local tall tale about flying manta rays (or sting rays; apparently they used to be called "stingarees" in some areas, and for all I know they still are). Specifically, flying ones. They don't actually fly.

They actually see one themselves, in the same place a man allegedly was taken away by one from.

Being suspicious materialists, they figure out they're balloons, being used to launch sounding rockets to spy on a nearby research station. Since the output of the research station is non-classified and hence published in full, this is a complete waste of time, but apparently the bad guys got confused.

Again, there's a fair amount of rifle play. Scotty, being an ex-marine, knows what he's doing in small-unit combat. (Which makes him considerably older than Rick, and it doesn't play out that way. I can see why Rick's parents would like an ex-marine hanging around with their kid, though, given the kinds of things they get up to.)