M24 rifle questions for Gunblr

He gun-toting side of tumblr, got some questions for you for a short story. I know very little about weaponry outside of basic gun safety, so bear with me.

I’m proofreading a pretty lousy short story for my fiction writing class and it ends with a PTSD addled vet (surprise!) shooting his four friends dead during an episode. And if it couldn’t get more tropey trite, he’s an ex-sniper. Character Jimmy carries around an m24 (that’s the only info on the rifle I get from the story, it’s called an m24) in his truck like a security blanket (it’s literally only covered by a blanket), whips it out and shoots his four friends dead from an undesignated distance, but contextually about a front yard’s distance away.

Here’s what I want to know: is this even possible? Can an “m24” shoot four people at such close range dead before they even can process it, would a “sniper” even do that?

From my understanding, rifles get less accurate at short distances, and logically I would think that for close range encounters, a “sniper” would carry a less wieldy gun. Wouldn’t their training have them react differently to getting stuck in what they think is a firefight? But am I completely wrong? I tried looking it up online and couldn’t find anything conclusive or helpful.

Any thoughts or tips to give this writer would be great, thanks!