"but it's a railgun" I think sir, that YOU do not know what a railgun is. All it is is a gun that fires a projectile using magnetic attraction and repulsion instead of gunpowder. It could be anything from a cannon, to a bb gun and it would still be a rail gun. I know an engineer who built a gauss gun once (Like a railgun, but more effective because it uses a magnetic coil instead of a rail) once, it stung when it shot you, but didn't even break skin.
Edit, railguns are configured in parallel rails to generate magnetism.
I am amazed that it takes so many shots to kill a vehicle. I don't know if they realize how a railgun works, or what it does, but it would punch a hole right through the armor of a mantis and straight through a spartan's chest.
Technically, the railgun's biggest advantage is that it uses a hardened projectile like iron or - more likely - steel, over softer metals like copper or lead. Reason it uses those metals over the traditional ones is because copper and lead aren't magnetic, and thus wouldn't be propelled by a railgun. The harder metals offer better penetration power because they're less likely to break apart on impact (for a good example, watch the episode of mythbusters where they were trying to get a bullet to ricochet back at the shooter).
Anyway, point is the railgun's power and penetration are actually solely dependent on how it's designed, strength of the magnets, and so on. Just because it is a railgun doesn't mean it will be super powerful or capable of penetrating hardened steel or titanium alloys with ease. Case and point a shotgun, for all the devastation it can wreak, cannot penetrate a standard Kevlar vest, let alone one that's reinforced with steel or ceramic plates.
Well at least SOMEONE can backup their post.
Why do I come here again? This place is so cynical...