How do I pick up this sniper rifle extended barrel?

Solution 1:

Basically, you can't do anything through the orange veil. (I think it's some sort of decontamination field?)

Fortunately, there are controls in the previous room that you can use to adjust where the orange veil falls within the room.

Simply activate the controls on either side of the door until the orange veil is on the far side of the sniper rifle mod, then activate them again to stop them. Go into the room, pick up the item, and voila!

You may need to move the veils again if you want to get out of the room, however.

Solution 2:

That was just a case of bad timing, it seems. The clean room sterilzer controls (located outside the room) move the barrier around. Simply pressing "deactivate" when the barrier is in a different position allows access to the sniper rifle extended barrel (and a door).