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.
You have to earn the infinite ammo first and it does not affect anything just like Resident Evil 5, it is the same thing.
You can actually only use the infinite ammo in a difficulty that has been completed already. (I.e... If completed on professional you can't equip in No Hope).
Can not use infinite ammo on no hope, but you can on all the rest of the difficulties regardless if you have previously played it or not
It is not possible to use infinite ammo on the first play-through of each campaign, and using it locks your ability to get achievements.
