Buying rifle under 21 in Missouri | ATF can intentionally delay for no reason?

I had just bought my first rifle and according to my understanding of the BSCA, the mandatory 3 day delay can also and is very commonly extended to 10 days even if I'm squeaky clean? What the fuck are they reviewing? I haven't ever even gotten a speeding ticket in my life. Or is it a gamble, where I could walk out with my rifle after or even before the 3 days are over with proceed?