This has been bugging me for weeks. For some reason, our Bombsquad seems to believe the bullets-as-currency mechanics is no longer present in the sequel to Metro 2033 (first mentioned a while ago and then restated as fact on this week's Bombcast) - that's not true! I don't know where they got this from.
Here's an article from two days ago that confirms it, if there is still any doubt. It might be used a little different but it's still there.
So bullets are still your ammo AND your currency to trade it in for food, weapons or more ammo.
edit: June 15 Interview on Rock Paper Shotgun:
RPS: So, first off, what’s the deal with the bullet economy this time around? Is it in or out? I’ve heard mixed things.
Beynon: It’s in. I think we were still discussing exactly how we were going implement that last time and, you know, all options are still on the table. But there’s quite a vocal fan response, which may be guiding part of our thinking. It’s staying in, though.
The thing we’re already working on is how we communicate that system better to the player. Metro’s full of a lot of quite complex game mechanics, and I think if there’s one thing we didn’t do very well in the first game, it’s we didn’t actually introduce those mechanics or explain them particularly. It was kind of, concept, wall of text on the screen, this is how this works. Have you read it? Press H, continue. If you missed it, it could really color your impression of the whole game. We think we can introduce those mechanics, including the trading system, a little bit smarter this time.
But no, it’s really important, we can’t lose any of those mechanics. That’s kind of a philosophy that runs through the game. We have absolutely no intention of westernizing or dumbing down Metro whatsoever. As you saw from the demo, if anything we’re adding more stuff in there, whether it’s the mask wipe mechanic, which is a whole additional thing that the player has to be mindful of and consider when they’re playing. Or an expanded weapon set, secondary fire modes, or attachments you can put on your weapon, secondary weapons… We now have a new stealth melee from behind. We’re also experimenting with putting in the option to knock out rather than kill if you approach people from behind. Loads of extra stuff is going in there.
So yeah. A very long-winded way of saying that we’re keeping the bullet-trading in and it’s going to be much, much better this time around. We’re not cutting anything.