I have settled on using Lapua brass and Hornady ELD-M 168 gr for my precision 308.  43.4 gr of varget tested best over many trials.  I am getting very good accuracy with new brass and even better accuracy with once or twice fired, but my velocities are wonky. I use a mandrel vs. expander since I was getting less consistent neck tension.

New brass, used a mandrel for consistent neck tension. Loaded to maximum length that will fit in my magazine which is a way back from the lands.  2630 FPS.  standard deviation has been 8-10 fps.  3/4" or smaller groups at 100 yards.

Same brass 1 or 2X fired, Full size die with a very .002" shoulder bump. Deburred neck but they were pretty OK. Mandrel again for neck tension.  Same load. Same bullet. Same depth.  Standard deviation is 20-35 depending on day.  Velocity slower - average 2545.  Here is the thing. It is so accurate it isn't funny. All holes touching at 100 yards.  20 bullets fired.

The only thing different is that it is a different cannister of varget, but same box and lot.  Any ideas WTH is going on?  The velocity variation and slower velocity is making my calculations a bit tougher.   I am not a precision reloader and just learning the basics, but this seems odd.

you can weigh the pills and brass to see if there is any deviation.  your brass will be effected with new to twice fired and even once fired.  you can get crazy with weighing everything but really depends on if you are shooting really long out to 1000 yards.  I don't think the powder is the issue although if the cans were stored differently there could be slight burn differences.  Also what is the round count of your rifle and your cleaning habits?

OP, new brass has not formed to your chamber, try bumping up your powder charge...As for the mandrel, if it gives you the size you want, use it..mandrel is the right way for finished neck tension and helps avoid moving the neck out of square as can happen with a button sizer when pulled out of the neck...Make sure you are not pushing the shoulder back any more than necessary, 0.002"-0.003" for bolt and 0.003"-0.004" for semi-auto....

How much difference between a never fired case and your chamber ?

If you don’t have tools to measure that:  I’d guess that you gained some case volume with the once fired brass.

I’d start with just bumping up your powder charge and see if SDs goes back to initial numbers

But,  on the other hand your rifle seems to be tack driving with the once fired brass…..I’d consider leaving it.

I would guess you are right about lower velocity from larger volume.  The improved accuracy is really nice, but the velocity deviation makes shooting out to 400, 600, 800 yards a bit less predictable. I can't think of any other variable. If I had more consistent velocities with the 1X, I wouldn't even consider touching a thing.

One thing about reloading is to establish a base line with good results using your current components.
The 308 is about as easy to load for as a 45ACP or 38 Special


Then doing controlled improvements test and see if things change?
Like seating depth?
Case processing?
You will always have baseline to return to.  But doing too many things all at once is fruitless.

Did switch primers and then you got strange results.

My case processing
Controlled bump with Redding Type S FL Die with a 0.339 bushing
The neck size with Redding Type S NK Die with a 0.336 bushing
Seated with a Forster Ultra Micrometer Seater

Components - Lapua brass, CCI BR2 and Varget has been very good out my 308 with a Sierra 175 SMK seated at magazine length.
I have loads with N540 and 185 Berger Juggernaut Tactical.
I had a nice load with 208 A-Max and RL17.  

This. If you loose accuracy then back off again.

1x brass has different capacity than new brass therefore pressure will change.

Are you wet tumbling?  I don't like the way wet tumbling removes all the slickness from brass and only dry tumble my precision brass.  This can cause neck tension issues

Annealing can help as well.

Also, are you absolutely certain of your powder charges?  It takes a good scale to be consistent.

Have you been recording the temperature on each range session?