Yeah, HoD is a railshooter with Zombies. It's also ridiculously hard. Especially 2, where it took an entire clip to put down a zombie and reloading was still "Shoot off-screen".
Basically, in the first two, you got a pistol. In one, it was fine because there were so few zombies hanging around, and a headshot was a sure kill. However, in two, there were far more, and headshots often only clipped off an ear or a nose. And Zombies barely flinched, too.
Then came three, with the shotgun. Best reload system so far, where you pump the shotgun to reload. It also packed enough power to drop most enemies in one shot again. However, in Boss fights, if you didn't drop the secondary damage meter far enough before the boss got to you, it would not only not take damage, but the secondary meter would FILL ALL THE WAY BACK UP! 3 practically required having someone to play with.
Then 4 gives you the TMP. The clip holds a lot more rounds, and you can now throw grenades. Which is nice, because in this one, there are A LOT MORE ZOMBIES. There's areas where you'll blow all three of your grenades, and STILL go through a few dozen clips. The damage meter problem is mostly fixed with the bosses- It's never stuck on one attack which is impossible for someone to do damage without a player 2, and most of the time the secodary meter doesn't reset after you get hit. However, the bosses also come alot faster, often hitting you before you can get off enough damage to make them flinch. However, it has the weakest ending of the lot: James blows up, the girl walks away, and Goldman tells you that people are still alive up north. And then you get text saying "Continued in House of the Dead 3". Wait, WTF? That's worse than a text ending! Come on!