Accepted answer
At 6.67 mg/mL a 1 mg dose is 0.15 mL — 15 units on a U-100 barrel — and no needle gauge changes that number. Gauge changes three other things: how long the draw takes, how much stays behind in the hub, and how much rubber you core out of the stopper. On the 27G scale a larger number is a finer needle, so a 27G needle is fine enough that a viscous solution draws slowly and a hurried draw pulls bubbles. If you are drawing 15 units at a time, the dead space matters more than the bore: a fixed-needle barrel loses microlitres, a luer hub loses tens of them, and at 6.67 mg/mL each microlitre is 6.67 µg.
The relevant detail is that this is a straightforward answer that people over-complicate because the numbering is counter-intuitive.
Fixed-needle insulin syringes are supplied in 29G to 31G and cannot be swapped for drawing, which is the trade-off against their much lower dead space.
Mechanically, drawing a viscous or foamy solution through a fine needle takes long enough that people rush the plunger, which causes more foaming. Using a wider drawing needle is the fix.
Coring risk as a function of needle gauge and insertion technique is documented in pharmacy compounding guidance.
Nothing here is medical advice.
Gauge numbers run backwards. Higher number, thinner needle.