New to rifle cartridge reloading and am a bit stumped.
Using a case gauge I have brass that drops in a bit to far meaning the shoulder is already pushed too far back.
Will a full length sizer move it out or do I just have to load them up and fire them to stretch it out?
A lot of my brass is just range pickup, not fired out of my rifles.

I’ll pick up brass also, some you can tell it’s kinda newer. That may be in the original box in the trash can, or bright & shiny, of the same make.

I have also left brass at the range, that I had no desire to reload. So, you never know what you may get.

Factory ammo has to fit any standard gun chamber. Reloads for a gun only really have to fit that chamber, though usually fit most others.

If the brass is good otherwise, should be fine to load it without worry. I wouldn’t be concerned about trying to extend the shoulder any.

I’m also usually a mid pressure level loader. When loading to modest pressure, one has more allowances with good safety margins.

I only use my case gauge after resizing so a FL sizer should help get you where you want to be.

A full length sizing die will only push the shoulder back towards the base. If you have a case where the shoulder is already further down the case than (shorter) a factory case shoulder, the only way to move the shoulder forward towards the bullet neck is to fire it.

^^^^^^^^
That, what he said there.