What is the length of the main bore?

The length of the main bore of the M777A2 Howitzer is indeed 200 inches. This measurement is important as it directly impacts the projectile's velocity and range. A longer bore allows for more propellant gases to act on the projectile for a longer duration, increasing its speed when fired, which translates to longer range capabilities. The design of the M777A2 reflects these engineering principles, aiming to optimize performance on the battlefield.

The other options do not reflect the actual specifications of the M777A2. Each of those lengths would not provide the same ballistic efficiency or performance characteristics that the correct length does, placing the M777A2 in a distinct category of artillery systems designed for modern warfare.