Throw ammo, shoot dudes.

-drop ammo boxes for teammates or in areas that you have a feeling youll be foxholing in, youll thank yourself later when youre mid firefight and running out of ammo 
-shoot at the chest. i made the mistake of trying to always go for headshots but its really not practical. especially with guns like the m16 (easily the best gun in the game imo although teh an94 owns in hardcore) the burst will pretty much guarantee all shots landed. 
-switch between using the acog and the red dot, theyre the best in spec1 
-always use magnum ammo. i dont agree with using body armor even though ive heard that in a 1v1 firefight, body armor always wins vs magnum ammo. i have my doubts about this claim but regardless, a lot of kills come from shooting people who prob havent even seen you so its important to be able to kill them as fast as possible so that you can engage your next target.  
-pick your battles and just have some common sense. dont try to engage snipers from long range and dont try to rampage into hordes of enemies. wait for reinforcements when youre alone 
 
-the m16 is seriously the best gun in the game. i can pull 4:1 and 5:1 ratios if im playing intelligently and pull off 3:1 ratios if i just feel like rampaging generally. i highly recommend you invest time in learning how to use it

as for the 40mm grenade, it is useful. i think the destructible walls is kind of a gimmick so i dont really use it for that. it is an insta kill against enemies if you can land a shot near them. i use it most if enemies are standing near walls or if im going downhill and i see enemies below me. i think hte 40mm shotgun is completely useless as assault rifles are good enough to take out enemies in cqb. smoke grenades are also somewhat useful, ex: if youre rushing the first base in arica harbor you are pretty exposed its good to land a few smoke grenades in your path. 

Going to chime in on the tactical use for 40mm grenades. (smokes have already been mentioned)
People seem to be really underestimating them.
First of all unlike CG engineers you basically have an almost infinite supply.
Secondly the time to fire is much faster then Gustav.
Thirdly reload time is faster.
 
On to tactical uses:
1. Breaching areas, houses, other objectives. Creating alternate routes. As an assault class you don't need no doors, you can make them anytime anywhere. It's not just a gimmick its a tactical advantage. You are not only creating more paths for yourself but for your team too.
2. Area suppression and area denial, no one is as effective imho as an assault in keeping large amounts of ground covered and suppressed as an assault. Trust me having grenades go off constantly in a specific area is a huge turnoff to many people, you can use it defencively or offencively to block routes or flush dudes from cover or allow more of your squad to move forward more easily becouse you keep lobing nades at the oposition keeping them more in check.
3. This flows right into Mcom denial. If I have clear sight on an Mcom and am standing a ways from it you can forget about arming it or disarming it without taking me out first. Point that trigger in an arc on an Mcom an laught as the other team tries to disarm it unless you get shot or knifed :P) when you learn maps and places where you can do it from cover it can mean the difference between taking that Mcom and loosing the game. Only class that comes close is medic with it's large ammo clip; recon and engineer reload too slow with their standard weaponry (sniper and CG) to keep that area locked down tighte then a nuns arse.
4. Oh you just hide behind cover did you, sorry but that bulding wall will not help you much :) Absolutely loooove killig people that use light cover. This means that to you as an assault much fewer things present a barrier either when the defence is dug in deep or the attackers are using cover to gain ground.
5. Ability to not care about ammo means you are much more free to send a grenade anywhere you feel like it unlike an engineer who can be hesitant to use those rockets for utilitarian purposes. to be clear here we are not talking about grenade spam here (it's not very effective due to large areas and low splash) but about breaching, suppression and other stuff. This basically suports all of the above points and makes them into more workable tactics then with just the CG
 
Sidenote:
Other stuff you might want to give a try is what I call going commando. What you do is leave any current  squad, pick any non assault weapon (I like G3) C4 explosives, extra C4, extra explosive dammage and try to go deep behind enemy lines. Try to use as much of a roundabout routes as you can, stay out of sight, only shoot when absolutely necessary,there are times when I let an enemy walk by if they did not see me, to preserve cover. Getting to that Mcom is the hardest part, thats why you don't want anyone spawning on you to prevent potentially alreting the enemy team. If you manage to get to the Mcom without gettign noticed you're set. Set those 6 C4s on the Mcom (Don't arm it!). Get some safe distance. Blow it. recharge your C4, plant and blow it again. Boom Mcom gone without that nasty alert that sends everyone and their mother running to it. Remnds me of Read Allert 2 commando unit every time I pull it off "A little C4 knockin on the door." :)
(this tactic works less on certain Mcoms due to exposure, also very good and aware defenders can ,ake it not very viable either)