I don´t know why the ammunition to be something so scarce in this game.  Perhaps this was a way to overcome some deficiency in combat.  But it is very annoying.     
 
 And by the way 5gb install in PS3 and and still a looong loading (and checking...) when start the game.

Are you playing in Co op?

" I don´t know why the ammunition to be something so scarce in this game.  Perhaps this was a way to overcome some deficiency in combat.  But it is very annoying.       And by the way 5gb install in PS3 and and still a looong loading (and checking...) when start the game. "

It´s can be true. The ammo box don´t popup very much from the background.  But still a pack of ammunition should not have only 20 rounds.

Yup, an ammo box fills your ammo to max... so depending on the ammo limit of the weapon, thats how much ammo you get.  By the way it also, maxes out all the weapons you have, which is way better than a lot of games that make you look for specific types of ammo. 
 
- marc

 I know. I said 20 rounds how a example. Played only the first chapter in decepticon campaign and the megatron main weapon has the 20 rounds limit. I often transform because the vehicle mode function like a weapon with ammo left and  wish it had other reasons to use the transformation during the fighting.  In the multiplayer that's better because you transform to  to escape from an enemy or chase him.    

5GB install is nothing compared to the epic installs in MGS4. There's the one the first time you put the disc in, and then one before each of the five chapters. However, if the developers force the install, it probably is for a good reason. 
 
And the only games I've played where not having much ammo can be considered a "good" thing would be survivor horror games, where the point of the game is that you feel underpowered and are supposed to be "afraid". So I can see how it'd negatively affect a third person shooter like this. 

I never had any ammo troubles.

I haven't had to much trouble with the ammo in the game and i just started the autobots section. When i started playing though with Megatron its is quite a pain due to his cannon only having twelve shots so you have to find an assualt rifle to make up for it. Breakdown starts with one.

I don't think I've had any ammo troubles at all....I'm surprised by how much I'm loving this game....I thought it was going to suck

Not having this problem at all.
Are you using the second weapon slot?

I had trouble finding ammo at first, but I had just been missing the ammo boxes or not seeing the bullets hovering on the ground. Once you realize what to look for, ammo isn't as scarce as you once thought.

Play Alan Wake, it'll teach you how to take looking for things to the extreme :)

I also found myself not every finding ammo but after a while I figured out they aren't always out in the open. Most of the ammo I find is in a different room than the battle or something like that.