Hey, I found it!
Had to use Google though... couldn't remember where I left the **** thing, and the forums search engine is a POS.

Anyway, before I get to that I'd just like to add a little to this infinite ammo thingy. Shobhits list specifically.
The only thing that I really have no argument with is the M&E, it's a total centerpiece of the SP and thus the devs glue the pistol in your hands in and around every cut-scene and corner. Doesn't really excuse infinite ammo, but it's probably the only truly justifiable reason. Even the takedowns, because you have many preventative measures, I.E. putting your pistol away, are not really a good reason.
Honestly I think it's the same reason pistols' where given infinite ammo in Vegas. It's because the games are made for maximum action with minimal consequence. Every single aspect of the game is handed to you with no strings attached. The lack of sound detection, the ridiculous commentary from the AI (giving you their position and plan), M&E firing through walls, the AI dies soon as you touch them (throwing them = death) and the naturally the ammo. It's auto reload and even if your assault rifle runs completely dry the game swaps to your pistol in a micro second (which is actually cool... the auto swap part, not the part where you toss your 25,000 dollar assault rifle into the gutter) It's all just excessive pampering in the name of not frustrating the player.
Also, shooting lights is pretty weak. Most of the game can be completed without shooting lights, and even if you did run dry, just tap 'A' next to a corpse and you'll pick up his weapon and ammo (even if you don't want to) and you're good to go.

Anyway, enough 'bout that. Here's the Reload writeup I did yonks ago
Something that has always been missing from Splinter Cell (and most other games, but that neither here nor there) is correct weapon reloading.
What I mean by this is if there is a bullet left in the chamber of the weapon then there is no need to load a new one just because you put a fresh magazine in.
In SCC this is most annoying with the assault rifles and SMGs. I for one like to be able to leave a bullet in the chamber for the sake of reloading faster, but this can't be done in SC, I strongly feel that it's time to fix this issue.

At the bottom right of the screen there would be [however many magz you're carrying] little magazine shaped thingies.
(The mag that is currently in your pistol would be significantly bigger then the others.)
Each one is like a white outline with little white bars inside of it symbolizing the bullets. As you shoot the little bars (bullets) deplete.

To reload:

Press and hold the reload ****on to do a tactical reload, this takes time but you will exchange the mag in your pistol for a fresh one thus keeping your unspent bullets, however if you tap the reload ****on you will reload much faster but drop your mag and loose the unspent bullets. (you can of course pick it up later)

Note: Your character will always put the fullest mag in his pistol when reloading.

Also, the back ****on on the 360 controller will be for your OPSAT or equivalent, so there you can micro manage your mags, I.E. get the bullets from two half empty mags and put them in one. (Think of it like combining herbs in Resi4) And sometimes you may only pick up a box of bullets, not a whole clip so you will have to unpack those bullets and put them in a mag before you can use them. This will also allow you to have lots in reserve, but considering the game WILL not pause when using the OPSAT/Advanced inventory it would be ill advised to do it in a firefight.

One last thing, if you try to pick up a magazine from the environment but are already carrying the maximum amount you will automatically exchange the least full one.
But if all your mags are fuller (or completely full) then picking up a new magazine is impossible. However, you can go into your advanced inventory and put some of your bullets into your reserve and then exchange the now empty clip. Your reserve supply would also have a believable limit.

P.S. Shotguns only have shells, hence there is no fussing around with magazines.
Tap the reload ****on to only put in one shell, or hold it to completely reload the weapon.