How to open Iron Doors??

[MeSako]

I see these thing all over the place.
And I managed to reach the room just outside the third boss. But one if those Iron Doors are keeping me out from the boss room.. I have been searching every area for a way in to that boss, but cant find it. I have found:
5 health packs..
3 maps..
4 bandoleers..
14 relics..
3 weapons.. (not counting start gun)
3 granades..
3 scrolls..
1 rappels gear..
AND no ****** way to open those **** doors!!

Help please!

[Ephires]

U need the C4 charges

[CriticaLink]

C4 only...kinda make me wonder why they even give you the choice of grenades.

[Gotta get that]

same question,how tu change the secondary???????

[MeSako]

...
Sigh..

Thanx guys.

[MadAsAHatter]

Had the same problem... couldn't find c4.