Hmmm, I think I know what you want.
Well, long story short, you would need to build a new model just for that, with also a specific animation to it, taking advantage of a certain MS3D bug (yeah, I use application bugs as features at times) and a bit of patience to pull off.
Now, personally I am thinking about 3 ways to pull out that effect:
- Easy: Make a cylinder, shape it slightly like a cone (but with no tip and no base), give it a proper "air burst" texture. No custom animation needed, just code it then so it goes into the desired direction (projectile physics and velocity), grows up slightly (drawscale) and fades away (scaleglow):
- Less easy: The exact effect you saw relative the megaton: make a cylinder with no central vertex on its bases, and then cut off everything except one of the bases.
From there, animate it to simulate a "grow", then in the script, set it to bParticles=True, set a smoke texture to it, and code it to animate, to fade away (scaleglow) and growing up slightly:
- Not so easy: Consists in a cylindrical shape which is then deformed to look hemispherical or "semi-oval", from there the animation would be the formation of that hemisphere outwards from a single point or circle... well, I cannot explain well this one, so here's a picture:
As for the bug (somewhat) to exploit here MS3D wise, is to scale up some animation joints: MS3D doesn't support such scaling "theoretically", but fact is if you use the "Scale All" function during animation mode, you can simply set the keyframe, copy the keyframe, undo the scale, paste the keyframe, done (I used this little hidden "bug-feature" myself quite often).