It's strange, some days I get no residual sting, some days I do. I started something new where I rub topical lidocaine 5% cream on the area I will inject and wait 20-30 mins. That has helped. I've also read some people add liquid lidocaine to their vial. GHK-Cu does not sting because of pH, it creates a histamine reaction. Anela recommends lidocaine with epinephrine in it. She replaces 1ml of BAC with 1ml of lidocaine 2% solution when mixing.
Also, are you aware that the distribution of nerves in our skin is uneven and it pays to give a tiny prick with the needle in the spot you plan to inject and if painful move to another area? I swab a larger area with the alcohol wipe to give me wiggle room.
Hope this helps!