AR 15 pistols of limited value, depending how short the pistol barrel is. For high ammo dependent on extreme velocity to be effective - such as the little .223/5.56 - reducing barrel length increasingly reduces velocity.
A 5 inch barrel for an AR 15 is 1800 fps. A 20 inch barrel is 2950 fps. There is no reason to go over 20 inches.
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The little .223/5.56 is very dependent on velocity for lethality. 2500 fps is considered the minimum for a "lethal wound channel" by military standards. In short, 10 inches it the minimum to reach that threshold.
There are also increasing reliability issues, flash and report volume issues, and barrel rise issues with extremely short barrels.
ALSO... NEVER convert an AR 15 rifle to pistol length. It is a MAJOR federal felony. If you WANT an AR15 pistol, build it yourself from a new lower - making certain it is NOT reported as for a rifle if you don't buy the lower in a private sale. That way, you can put any upper (barrel) on it of any length. A "pistol" AR15 may be converted to a rifle, but not visa versa. DO NOT trust the store selling the AR15 pistol isn't selling you one that the feds were told it is a rifle.