I have a AR15 platform and a 10.5" 300 blackout upper, 16" 5.56 upper, 18" 450 bushmaster upper, and a 20" 6.5 Grendel upper. What would be good purposes for all of these configurations? What type of ammo should I have (including gain size), what would be the effective range for each in yards and what kind of scope should I use for each?

You have a versatile set of AR-15 uppers with different calibers and barrel lengths, each suited for specific purposes. Here's a breakdown of their potential uses, recommended ammo, effective ranges, and suitable optics:

1. 10.5" 300 Blackout Upper:

   • Purpose: Home defense, close-quarters combat, and hunting in shorter-range environments.
   • Recommended Ammo: 110-125 grain supersonic or 220 grain subsonic ammunition for versatility.
   • Effective Range: Up to 200-300 yards.
   • Optics: Red dot sights, holographic sights, or low-power variable optics (1-6x or 1-4x).

2. 16" 5.56 Upper:

   • Purpose: General-purpose rifle for target shooting, plinking, and self-defense.
   • Recommended Ammo: 55 or 62 grain FMJ for target practice and 75-77 grain for accuracy.
   • Effective Range: 400-600 yards depending on ammunition and shooter skill.
   • Optics: A wide range of optics is suitable, from red dots to low- to mid-range magnified scopes (1-6x or 3-9x).

3. 18" 450 Bushmaster Upper:

   • Purpose: Hunting medium to large game at moderate ranges.
   • Recommended Ammo: 250-300 grain expanding bullets for hunting.
   • Effective Range: Effective up to 200-250 yards.
   • Optics: A low- to mid-range variable optic (3-9x or 2-7x) or a red dot with a magnifier.

4. 20" 6.5 Grendel Upper:

   • Purpose: Precision shooting, target shooting, and hunting at longer ranges.
   • Recommended Ammo: 120-130 grain for target shooting and 129-140 grain for hunting.
   • Effective Range: 600-800 yards or more, depending on shooter skill and ammunition.
   • Optics: A high-quality variable optic with magnification (4-16x or 5-25x) or a dedicated precision scope.

Remember that your personal preferences and specific use cases may vary, so it's essential to test different ammunition brands and loads to find what works best with each upper. Additionally, invest in quality optics that match the intended purpose of each upper, and consider factors like durability, reticle type, and turret adjustments.

Always adhere to local laws and regulations regarding firearm ownership and use, and prioritize safety and responsible firearm handling at all times.