For non-alive payloads you can accelerate at >10g and get a comfortably short rail to fire the bullet from. For alive payloads, the rail becomes longer. The a

For non-alive payloads you can accelerate at >10g and get a comfortably short rail to fire the bullet from. For alive payloads, the rail becomes longer. The advantage is that on the moon the low gravity and no atmosphere make megastructures like this easier to build. Energy is also no concern either, you have pure unfiltered sunlight for about 15 days each month on each side of the moon.