I guess Ill elaborate my disparaging remark against white folks...
I fail to really see this one as a big deal. The .300 AAC Blackout is the formal name...also known as 300 BLK or 300 AAC. But when you see 300BO I doubt anyone is at all confused as to what it is. Most people I have talked to about the round in casual conversation just refer to it as 300 black out, so shortening it to 300BO makes sense. Not technically right, but not worth snickering behind peoples backs about. Does it show a slight lack in familiarity with the caliber? Perhaps. But it isn't like calling a magazine a clip, or all 1911's a Colt, or all revolvers a 6 shooter.
