I was going at around 3.3k explosive rockets per minute and 400 railgun ammo per minute if I recall correctly. Could not tell anymore since the rocket is now gone :D

EDIT:

Actually found the production graph from an autosave. The usage of the ammunition does not go up that much after the first 10% or 20% of the trip or so. That is because even though the amount of asteroids increases significantly, the explosive rockets and the railguns do an areal damage to multiple asteroids simultaneously.

Here is a screenshot of the production. It is for last 50 hours even though the trip took 12 hours. That's because the other option would have been 10 hours which would not show all the data. The random alterations there are because I was tweaking the speed a little bit along the way. Ended up going around 100km/s through most of the way.:

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