The Kel-Tec P3AT 380 fits nicely in the shirt pocket.
I have tried carrying a lot of different handguns, but the best I have found is a Kel-Tec P3AT 380 loaded with 6.2 gr Power Pistol 90 gr Gold Dot, chronographs at 1100 fps, with Kel-Tec pocket clip held with 4-40 acorn nut, DESANTIS M44BJG5Z0 holster, and ArmaLaser Kel-Tec P32/P3AT TR1 Laser.
The P3AT is 6 ounces, 10 ounces loaded.