Is the only difference between a superlight and an M4 contour is that the superlights entire forward portion is the same diameter as the small diameter inside the M203 cut on the M4?

Yes, the 'Superlight barrel' is basically the old A1 diameter.  Also the regular M4 barrel is only 14.5 although they do make a 16 inch version.

yeah i think so i have both but not at them right now but i know that it is skinny muzzle to sight base then from there it tapers down again all the way to the nut

So, to clarify.  When machining the M203 cut on an m4 barrrel, if they did the whole forward portion that same diameter, it would end up being a superlite?

If i understand it correctly, yes.  But I'm no expert.

From the Bushmaster website:

"Diameter tapers from .670 inches at the chamber end to .600, then steps up to .625 at the sight base, and tapers to .570 from the sight base to the muzzle - similar to the original Vietnam era A1 barrel profile."

I believe the M4 barrel is a little thicker than 0.600 under the handguards, so just reducing from the FSB forward wouldn't make an M4 barrel identical to the superlight.  Pics attached for visual reference.





Edited to add:  Silly me, I have a Bushy M4 barrel in my safe, so I measured it under the handguards:  0.640" & the M4 cut measures out at 0.610".