Thank you for reaching out, Blake! Either of the two cartridges you mention would provide effective performance for self-defense out of your P365. Naturally you should expect a slower muzzle velocity out of your 3.1" barrel, as manufacturers typically use a longer barrel to publish their official ballistic data – but the Gold Dot and G2 are both designed for terminal expansion over a range of velocities, encluding those on the lower end of the spectrum. It is our understanding that a heavier bullet gives a shorter barrel more time to complete its combustion cycle, so either a 124 or 147 grain should promote good functionality in a P365.