A somewhat silly response, but to me ATACMS sounds a lot like the plural of atack'em which is short for "atack them" in colloquial "American"

And HIMARS sounds like you are sending something to Mars. Quite the thing to name an artillery platform :)

For some reasons Americans like acronyms that spell words. Like SAVE Act (Safeguard American Voter Eligibility) or PATRIOT act (Providing Appropriate Tools Required to Intercept and Obstruct Terrorism)