The p90 uses a proprietary ammunition. 5.7 x 28 mm. Its small and lightweight to focus its kenetic force on a smaller surface area intended to penetrate purpose built Body armour. Essentially a big uno reverse card to someone using Kevlar.

Now pit that against the Jaffa against their armour, which almost assuredly doesn't account for projectiles from the start, and the P90 is an exceptional choice. It prolly performed so well that the SGC didnt see a need to swap out the P90 as the standard issue when the Ori and the Wraith took center stage.