What classifies them as 2nd's, or Blem's?
Lots of things, but I imagine it may vary from make to make. Sierra seconds can be made up of a variety of things, ranging from something being out of spec (which isn’t always a problem) to something being truly wrong with them. This may be something as minor as a slight scratch left during sizing, or in the case off spitzers, tips and bonnets that don’t tumble off properly in the final wash and polish operation. When a bullet press is set up initially, or has a major change during a run, it is normal (used to be, anyway) for the first 500-1,000 bullets to be tossed into seconds as th machine is still warming up and settling in. Nothing whatsoever wrong with them, just normal SOP in setting a press up. Or it can be something as trivial as a cosmetic issue, like jackets being somewhat darker than they normally should be after final polish. Customers complain about such things, so it’s often worthwhile to dump them rather than field th complaints down the road. The problem is, you generally won’t know what the bullets were rejected for, and once they go into seconds, no attempt is made to segregate them by lot, cause of rejection, etc.. So expect to do some hand sorting if you buy seconds.
As to what may be available at any given time, there’s two primary factors that determine just how likely you are to find a given bullet; volume, and difficulty. If a bullet is always being run, like say, .308” 168 or 175 grain HPBTs, and the tolerances ar very tight, you’re pretty well assured to be able to find them in stock. For something like a .45 cal 230 grain FMJ, it’s an easy bullet to run, tolerances aren’t as strict as with the Match bullets, and they don’t run nearly as often. As a result, they can be chancy to find in stock, so grab while the grabbing is good.
I actually shot “seconds” for probably my first ten years or so at Camp Perry for the Nationals, without ever having any sort of problem. Any nines (or worse) that showed up we’re my own doing, and nothing I could blame on the bullets. I did have the luxury of knowing why a particular batch had been downgraded and not boxed up as firsts, or I probably wouldn’t have been willing to use the seconds for actual competition. Perfectly fin for practice, though. In the end, it’s always your call.