Something happens to jacketed bullets when they are left in bare necks too long. They become bonded, stuck, whatever and this causes ES to go up. Some people call that cold welding. It is readily and easily detected by trying to seat these bullets a little deeper. You hear and feel a pop. That pop is the bond breaking.
If you introduce a lubricant in between the bullet and the neck, you will prevent this bond from forming and you will be able to seat bullets deeper or pull the bullets out smoothly and easily.
People who deny the above are fucken retards.
How much does the ES go up? How long does this process take? 1 week? 1 month? 1 year? 1 decade? Does ES gradually get worse throughout time, or does it "settle" at one spot? If you let ammo set for 1 year and compare it to ammo that was made 6 months ago, versus ones that were made yesterday, will the ES correspondingly be higher with the age of the ammo? If so, by how much?
I don't clean my brass to sparkling new - there's some residue still left inside the case mouth. I dry tumble with rice for ~1 hour, I don't have any insane cleaning regime, and I don't understand the vanity of shiny new clean brass. I've personally never noticed an increase in ES in my reloads that correspond with age. I can take my 6BRA reloads from 1+ years ago, and they will have an SD between ~3-5 fps.
I'm not sure if FGMM would lube their necks, but FGMM ammo that's almost a decade old shoots in the single digit SD's (~6) out of my TacOps rifle. Perhaps FGMM lubes the bullets/neck, but even if they did, you would expect that lube to degrade over time.