Okay, so I was looking around the wiki, and I had found NO special Assault Carbines! I could only see the All American, and that's the marksman carbine special, and it runs off of a different ammo!

I also need help on how to find the key to the armory in Vault 34.

The only thing you can do with an Assault Carbine is give it the extended mags mod. I wouldn't worry too much about finding a "gonna keep you forever" Assault Carbine though. I switched over to Light Machine Guns as soon as I found one and never looked back.
As for Vault 34... that's a toughie. You need to open the Armory door via the computer terminal in the Overseer's office. Do a search on this forum for Vault 34 and a pretty decent walkthrough will pop up. I personally loved that level, and the reward at the end is well-worth it.

that valult is murder but the all american is worth it in the end just make sure u bring the space suit and alot of rad away :flamethrower:

This will be hard to explain but here it goes. Head to the Overseer's door and it is locked. Backtrack until you see a broken pipe in a off corridor, there will be another corridor that leads down into some water take that route and once in the water look left and there is a corpse to loot. Head back up toward the Overseers office and just before that there is a terminal that you can now use. Pick the system option and drain water. Now head back to where you looted the terminal password and the water is gone and a new path is open. Keep going until you see another broken water pipe this time it is right in the main hall. Again there should be a path down into water near there repeat and loot another corpse for the terminal password. Once you get it head back out and find another computer that opens the Overseers door. Then backtrack all the back and in his office open up the armor then you are set. Best I can do hope it helps.

My first attempt at Vault 34 was successful, but I had to use a ton of meds and stimpacks. I wasn't happy with that, so i redid it, but used a few stealthboys I got from the Repconn Nightkins, and I swarmed the place in a matter of minutes, using very few meds and 1 stimpack.