I have the explosive shotgun and it looks to me like it actually fires some sort of shells instead of a scatter-shot pattern, - will every weapon I find with the "Explosive" special ability just lob shells like that? Also do the Broadcaster type weapons just have the ability to kill everything, at the sacrifice of range?

I don't think there are many or any other Explosive type weapons in the game, so I couldn't say.  It looks like that shotty shoots a mine at the enemies.  Broadcasters were described as "flamethrowers" in the GB quicklook.

Broadcasters will eventually kill everything, but they're not much good when it comes to heavily armoured beasties like Big Willie or Breakers, and they're totally useless for Void Droppers. They are best for small numbers of swarming bugs like the Knobs, but the limited range can be a pain - it means you have to dash around the battlefield to intercept the swarm before it gets to your turret or base. They're still pretty useful for certain maps, but there are better alternatives IMHO.

I have a few questions of my own (apologies if they've been answered elsewhere :))

Can anyone tell me how the ricochet weapons work? Is the ricochet effect random or can I make it happen more effectively by aiming a particular way? 
Do any of the Marine items affect gameplay or are they purely cosmetic?

@Binaryfart: No idea on the ricochet, I have not tried one of those sniper rifles yet. But the Marine items are purely consmetic

@Binaryfart said:

Broadcasters will eventually kill everything, but they're not much good when it comes to heavily armoured beasties like Big Willie or Breakers, and they're totally useless for Void Droppers. They are best for small numbers of swarming bugs like the Knobs, but the limited range can be a pain - it means you have to dash around the battlefield to intercept the swarm before it gets to your turret or base. They're still pretty useful for certain maps, but there are better alternatives IMHO.

I have a few questions of my own (apologies if they've been answered elsewhere :))

Can anyone tell me how the ricochet weapons work? Is the ricochet effect random or can I make it happen more effectively by aiming a particular way?
Do any of the Marine items affect gameplay or are they purely cosmetic?

Ricochet seems to happen when you hit any hard surface (like the ground or Breakers hard shell), and maybe more often than that. I found a really effective tactic is to shoot at the ground near a swarm of enemies when they are coming at you, and the ricochet will bounce up and hit em. Pretty sure that's how I got the achievement.