Even if you find the case you're looking for, what's the relevance? You've agreed that Congress has maintained the tax on guns, so obviously the ruling you're looking for didn't apply to that.
The Supreme Court, in Sonzinsky v. United States, 1937, held that the National Firearms Act was sustainable only as a revenue raising measure. Congress (and the states) have blatantly imposed taxes to discourage use -- "sin" taxes come to mind. I'm sure the taxes on cigarettes have prohibited many from smoking. Yet all are constitutional.
(Perhaps you're thinking of the 1991 case, US v. Rock Island Armory, Inc., where the court ruled the tax on machine guns unconstitutional. This resulted because the government completely ceased accepting registrations for machine-guns upon passage of section 922(o), thus removing the "constitutional legitimacy of registration as an aid to tax collection.")
It would be nice if the congress did what the laws mandate it to do, but that only happens in the Bizarro World.