Are there any penalties for using infinite ammo?

When you select a campaign, you also get to select some game mode settings (the bit with network play options). One of the options here is to enable infinite ammo - which is available in offline and online modes.

What does this mean? Can you use infinite ammo any time, or does this mean you can use an "infinite ammo" unlock reward once you have it? And more importantly, are there any disadvantages to enabling this mode? (e.g. prevents trophies, prevent completion progress)

The option at the game setup is just there to allow you to choose whether or not to enable it for those players (i.e., in co-op) that have already bought it.

You will need to complete all 4 campaigns at least once to unlock the ability to purchase infinite ammo for your weapons. You will have to buy infinite ammo for each weapon separately.

The costs for each weapon are as follows:

Handgun: 79,000 skill points

Shotgun: 89,000 skill points

Magnum: 99,000 skill points

Sniper Rifle: 79,000 skill points

Machine Pistol: 89,000 skill points

Assault Rifle: 89,000 skill points

Grenade Launcher: 99,000 skill points

Crossbow: 79,000 skill points

Considering how difficult it is to unlock and obtain, I'm pretty sure it doesn't turn off achievements (RE5 did the same thing with infinite ammo). It's pretty balanced since infinite ammo has to take up one of your skills in your skill set, so you can only have infinite ammo for three types of weapons.