What to Know Before Buying a 5.5 Inch 300 Blackout Pistol That’s why the 5.5'' 300 blackout pistol has become such a popular topic among builders and shooters. On paper, it sounds like a very specific setup. It fills a niche that neither a standard rifle nor a typical pistol caliber AR really covers. It’s a compact platform that still delivers rifle-caliber performance, which is exactly what many builders are looking for.