There's a little trick that i used when i played through on maximum difficulty which may help you out.
**SPOILER WARNING**
In the little ammunition storage room (the last one before you get to the big doors) there is a chair. Get you pistol out and shoot the chair to the big door, open the door and place the chair in the line of the door so that it can't close

It leaves it open for you to get more ammunition.
If you want to do it the non-hacky kind of way then there is something else you can do. Collect all your ammo from the storage room and go over to the big doors. Open the first one and stay close enough so that it doesn't close on you. Using the sub with the square eye sight (can't remember the name of it) aim in and shoot the pesky little trigens with guns. You may want to use Cry-Vision/binoculars to see where they are. After killing them, move around so that you attract the attention of the mahoosive ones. Once one of them sees you then the rest will follow. Now get out the chain gun and, moving around as much as possible, shoot each one in the head as they come to you. Use this method first before using the chair then you'll feel better about yourself when you beat them
That should get you past that bit, but i found the next step even harder

You have to persevere and finish it so you can play it again, it's such an awesome game.