Keep in mind a 50cal from a sniper rifle (same round/casing as from WW2, different gun) can penetrate 2 separate 1/2 in steel plates placed 3 feet from eath other, when the shot is fired from 1 mile away (specifically measured).
In closer? It's got even more punch.
Like I said, M16s could tear through the thing. It was the basic minimal armor to protect the crew, not to stop any rounds from any guns larger than those carried by foot soldiers.
Um... I'll do some digging on those stats (2 separate 1/2 in steel plates placed 3 feet apart), but I'm pretty sure it cant do that from 1 mile out. With a special type of projectile (a step up from the typical FMJ or AP bullet), then perhaps. Also, the .50 cal ammo in WWII is not the same that is being used today. The propellant is different and the bullet is actually 5 gr lighter, meaning that today the .50 cal has a slighly less flash report and slightly higher velocity.
And... the idea of the M18 would not be to charge in and get within punching distance of an enemy tank (or auto/manned ack, or even M16 range), it would be to take on enemy tanks from as far away as possible with a good gun, high rate of fire (for a tank cannon anyways), and high menouverability (shoot-n-scoot).
If the M16 can in fact take out the hull of a M18 from 1600 yards out, then even for M16 (or a wirby) to find the M18, get a bearing on it with its guns, get enough hits on it to do said dmg... isnt easy. Anyone who would bring the M18 in close is just asking for trouble just as you mentioned in a prior post (close infanty support = a destroyed M18).