Individual or Grouped
The damage you do is simply a function of the number of C4 you set off, whether you set them off individually or in groups.
Count
In his excellent analysis of BFBC2 damage, Den Kirkson analyzed the code and found that C4 does 290 damage at 0-1 meters, and then does a linear drop-off to zero damage at 8 meters. (A human without body armor in the game has 100 hit points, a tank has 1250.)
Kirkson doesn't document, and I don't know, exactly how much health an MCOM station has. But it takes 12 C4 to take out an MCOM station, so the health is somewhere in the neighborhood of 3400.
Which is a lot.
MCOM Collapse
Because of the MCOM's massive health, it's always better to take out a station indirectly by taking out the building when possible. This guide gives details on how many C4 it takes and where to place them on some of the Rush buildings. It takes much less C4 than taking out the MCOM directly.
Placement
It's fine to place your C4 within 1 meter of the MCOM, but be sure no indestructible obstructions will interfere with the blast radius.
Personally, I like putting them on the ceiling since enemies are least likely to look there when running in to disarm the MCOM.
Sequence
- Place the C4
- Arm the MCOM
- Run and hide
- Toss a motion sensor if you have one
- When enemies come to disarm the MCOM, detonate
I used to arm the MCOM before setting the C4. That was... um... a really bad idea. Gives people extra time to find you before you hide. Don't do that.
Loadout
People often think of C4 as a recon thing, but if you want to be a C4 thug you should really play assault and use one of the "all classes" weapons, which will let you pick C4 as your add-on. Your ammo crate gives you an unlimited supply of C4.
Regardless of which kit you play, if you take the explosives perk you'll be able to carry 6 C4 at once, enough to take out many buildings without re-arming.
Out of Bounds
One favorite tactic of... certain people... is to have a friend load up a UAV with C4 and pilot it right to the enemy's MCOM. Do this a couple of times and you can take out the station with no risk.
Except for the risk of getting cursed out, kicked, or even banned, because it's an incredibly lame tactic.
So don't do that.
Use Mines Instead
The engineer's mines have the advantage of staying around and active after his death. A recent poll in the BFBC2 forums shows support for this method.
Best Answer
If you go your steam library and select "Add a game"->"Add a non-steam game" then find the executable for BF:BC2, this will show your friends when you are playing BC2. You will also be able to access the in-game overlay for PMing your steam friends and using the browser. Sadly, it won't show any details about the server you are on, just that you are playing it.
This is the best you can currently integrate non-steam games into steam (games that don't have a steam-activatable key, at least).