I ran a search on it, but I didn't find much. Is it at all possible? (I'd like to make a sort of trail for a tank shell)

Yes, but not in the sense of trails like MOPixels have (sadly. I've tried to get this to work but the atoms don't want to play ;-;). Use an AEmitter, it's just a MOSRotating that can emit any other MO from itself.

Emitters don't inherit velocity, ahem, so if you want a smooth line trail like those of MOPixels on other stuff, it might be best to use Lua to put a MOPixel at the same position and velocity as the object. You could then either keep that one MOPixel in place and replace it if it's gone, or have it be short-lived and replace it on every update.

Emitters do Inherit the DIRECTION of the emitter's velocity though.

You could make it shoot the emitter with a spread = 0 and make the MaxVelocity and the MinVelocity the same as the FireVelocity..

I havent tried this though so I could be entirely wrong.

Thank you. This is very enlightening. I wonder if Data will ever implement trails on things other than MOPs

You're right, but if gravity, air resistance, a ricochet, or something changed the direction or speed, then the trail would be somewhat off. With OrientToVel, you can make it always fit the direction, but still not the velocity.

I'd wager a guess: no.