Ive unlocked the room, using lockpick, and the plasma rifle isnt floating, its just leaning up the bottom of the case..?
I cant get it, and when i try and use explosives to move it, it goes deeper into the floor...
How could i get this ?
Damn bugs... ><

Note that this will disable steam achievements if you do it.

PC? If so, open the console, and type 'twf' to switch to wireframe. Click on the rifle and then, WITHOUT moving your mouse, roll the mousewheel. Watch the top of the screen.

When you get the rifle selected, type into the console 'moveto player' and then 'twf' to switch out from wireframe mode. exit the console. It should now be lying at your feet.

... if you're on an xbox or ps3... I'm sorry for your loss.

Thanks for the information!
Though, im on PS3, dont have a high powered PC, just a high powered laptop, that cant run powerful games x)

svcks tbh, shouldn't happen, but still...

In that case I'm afraid the best chance you have is reload a save from before you entered the building. At least you know what to do now. Otherwise just give up and move on. I hear there's a rather nice laser rifle prototype at the bottom of a vault somewhere.....