This is good for your first flash animation. Keep practicing. ;)
I've never heard of Metal Slug, so if there were any jokes in here I missed them completely, but it looked nice enough. The only thing I thought was off was how empty some of the scenes were. Like when the guy was demonstrating all the weapons? The only suggestion I can make is that you lower the screen size so your stage doesn't look quite so empty.
But, the animation was very nice and I like the overall product. Nice job =)
It was pointless...
...but it did have some humorus affects, my favorite part is when the rifle got jammed.