As said in the other forums you've posted this question in, I use 18 gauge for holding power, colored head nails just because, and I vary the air pressure to control depth of pierce to avoid canyons or splits in the material being fastened.
I adjust pressure on scrap, and have a regulator immediately at the work site, not at the compressor. It goes compressor, 100 feet of line (which serves as an extra "tank"), then regulator, then a coil hose (those yellow helix things that are supposed to stretch out to fifty feet but I rarely stretch mine out beyond 10 feet), then IRAX swivel joint or equivalent, then pneumatic brad nail gun (mine is a Bostich B135). The ammo brads have T heads, and I align the head of the T with the grain of the wood, if tacking stain grade trim.
The randomization of the grain line, combined with the pre painted ammo, makes the fasteners disappear, even without finishing.