If you want to build a custom pistol caliber AR, there is one major decision to make first. Do you want to build it in 9mm or .45 ACP. There are differences and benefits to both. Lets Review them and go from there.
9mm, which is one of the most commonly used rounds today in the world, is always a favorite.The benefit of the 9mm is the controllability and the capacity, not to mention that these days it is fairly inexpensive to shoot.Some believe the 9mm is better for home defense because it will not penetrate as much as the .45 acp The 9mm also has less felt recoil than the .45 acp. 9mm has been used in our Military for over 30 years, and continues to be used today.
For the .45 ACP, the round has survived two World Wars, Korea, Vietnam, and has been used in our military up to the 1980s. Famous for major stopping power. It does have more penetrating power than the 9mm, and some believe bigger is better.
It is personal preference and either one makes a good choice for building an AR pistol or rifle.