I just got 1000 new (never been fired) LC 223 brass and when I measure it with a case gauge, some of the brass is slightly below the low step. There is no way to lengthen the brass except to fire it in my AR-15. I know that too long of brass can be shorten by the sizing die and trimming it, but how do you lengthen it, or should I even worry?

Thanks, Gary

Gary, the "trim to" figure is an arbitary number that really means very little. Unlike the "max length" which is based on industry standards for chamber diminions.
If it were me, I'd segregate the ones that are too short and trim them to a uniform length matching the shortest one. And I would shoot those as a "unit". The ones that fall within the parameters, I'd trim to match the shortest one and use those as a seperate unit.
Shooting them in an AR, it probably won't make any difference if you just load 'em up and shoot 'em.
There shouldn't be any difference in accuracy.

Don't worry about the ones that are a bit short, except if you are going to crimp bullets. I don't mean to encourage crimping of .223's, as I've never found that necessary.

You'll probably need to trim after your first firing, so it won't be a problem for long.