1.) How much accuracy/velocity/distance is gained by going from a 16" barrel to a 20" barrel?
- If both are HBARs
- If the 16" is not an HBAR but the 20" is?

2.) If you swap uppers can you use the same bolt/carrier between them or is it best ot have a dedicated bolt/carrier for each upper?

1. From the ammo oracle:

M193 should give you around 3200-3250 FPS from the muzzle of a 20" weapon and around 3150 from a 16" weapon.

M855 should give you around 3050-3100 FPS from the muzzle of a 20" weapon and around 2950-3000 from a 16" weapon.

2. It is recommended to keep a dedicated bolt for each barrel.