I'm trying to recall the name of a sci fi novel where a wimpy, sickly son swaps bodies with his tough military father. I think it had a picture of some kind of futuristic gun on the cover.
Was it part of this series of young adult novels?
From the wiki link above:
Help! I'm Trapped... is a series of 17 books written by Todd Strasser, published by Scholastic Press. With worldwide sales of over 10 million copies, the plots mainly center around a group of children and a machine that has the power to switch bodies.
The characters include Jake Sherman, who is usually the one "trapped in" someone else's body, his sister Jessica, his two friends Andy and Josh, and their science teacher who made the machine. Recurring characters include Jake's parents and dog and the principal.
The first of the series, Help! I'm Trapped in my Teacher's Body, was published in 1993.