Does zeroing with 115 vs. 147gr matter big time? I mean, I shoot 115 range ammo, so I should use that to zero right? And know where my shots go when I use HD ammo.
Glock recommends shooting 124 gr or heavier ammo in their 9mm pistols. Heavier projectiles not only increase accuracy (more mass = less susceptible to forces of wind, etc.), they also create more force inside the gun, helping it to cycle as designed.
That's the theory, anyway. YMMV.
Glock was having ejection issues in most of their Gen 4 9mm pistols. The direction of the spent brass when ejected was sporadic, and it often flew into the shooter's face! We experienced this, so I contacted Glock. They suggested using the recommended bullet weight to see if that helped. I found out this was a widely known issue caused by the redesigned ejector and trigger parts for that gen. Eventually Glock started putting a newer ejector in these pistols, but never did a recall of the previous parts.
I replaced the trigger and associated parts with an upgrade kit, and the problem was about 90% fixed. Still got an occasional brass near the face, but it was rare.
Anyway, that's how I was informed of the 124 gr bullet recommendation directly from Glock.