Ok, i have done a bit of reasearch into this one.
animationgap = This is the time between each “animation” of the Standard Plasma sprite. The higher the value, the longer between each frame.
frames = I think that this is a deactivated value. You can even remove it from the plasma weapon all together and the game will not crash.
At a guess i would say that early on in the development the plasma bullet was pre animated - for example a dds file had 4 images of the plasma bullet and it would cycle through them using the animationgap as the interval. So if you were to replace the plasma bullet with a more animated image you would specify it had 16 frames.
In the end that frames was to be used for is a secret only cliffski knows . . .