On the subject of missile trails, I believe that those are actually the plasma torches of the ship's fusion drives.
Various vessels and engineerings have different drives, so thus the different varieties in "trails."
As for the Maxim, it's stated it is a high velocity weapon and it accelerates the projectile through the barrel, it is
never stated that the slugs are accelerated to relativistic speed such as in a rail weapon.
My assumption is that in this case the shape of the shells and method of acceleration makes it more efficient than a relativistic weapon. (also the amount of ammunition within the cannon means that the only thing counted is the energy used logistically)
The Avenger cannon in the first FreeSpace is similar, save the "pulse weapon effect" is actually thousands of small bullets flying out together in a close pattern.
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AceStaff member FreeSpace Watch
http://freespace.volitionwatch.com/