"Use .50 caliber or above and preferably aim for the head..."

If only he would have done that in the testing stages, in that old factory unit. It would have shaved a good hour off the runtime on this pos. :-)

That was stupid anyway. Caliber isn't a deciding factor on its own. Bullet weight, bullet velocity, and bullet construction are the most important factors when it comes to defeating armor. If anything, larger caliber bullets are less effective at defeating armor than smaller calibers, due to increased frontal area, which reduces the amount of force per unit of area exerted on the target, all else being equal. Is it easier to penetrate something with a needle or with a thimble?

The characters talked as though ".50 caliber" refers to just one type of cartridge. In reality it could refer to anything from e.g., .50 GI to e.g., .50 BMG, and there's a massive difference between those two. A .50 GI is a pistol cartridge which is marginally more powerful than a .45 ACP, and has all it can do to penetrate a road sign. Standard soft body armor would stop it easily. A .50 BMG is about 45 times more powerful and would go through "Robocop" like a hot knife through butter. A .416 Barrett, which is a shortened and necked down .50 BMG, would have no problem going through Robocop either, despite it being smaller than the "magical" .50 caliber.

There are plenty of other sub .50 caliber rifle cartridges that would easily go through Robocop as well. Even a common-as-dirt .30-06 Springfield ("only" .30 caliber) with armor-piercing bullets would likely make it through, given that it can penetrate about 1¼" thick mild steel (~130 brinell hardness) or about ½" thick steel armor (~500 brinell hardness).

Also, were there ever any mention about armoring between his face and brain(however that would fit)?
Regular small arms fire under his visor should do the trick, yet they talk about 50 cal but still to the head area