.50 is generally the line between 2f and 3f, so .50 and smaller, 3f. But I've also seen the very next sentence also say, ".50 and larger, use 2f"... so I'm guessing it makes no real difference. Using comparable volumes, you just get a little less steam behind the bullet with 2f. If 2f is available and 3f isn't, it certainly isn't a deal breaker - I buy the 2f.