Cost is also an important factor when choosing ammunition. 9mm ammo is generally more affordable than many other types. By opting for a 9mm AR upper, you can save money on ammo, allowing for more practice without spending a lot. This makes the 9mm AR upper a practical choice for regular shooters. Cost is also an important factor when choosing ammunition. 9mm ammo is generally more affordable than many other types. By opting for a 9mm AR upper, you can save money on ammo, allowing for more practice without spending a lot. This makes the 9mm AR upper a practical choice for regular shooters.