Fortunate for you, no need to touch the scripting. Each object, including NPC or a weapon, has it's ID you can find in their own tabs. Once you track, for example a weapon, you want more unique, right click on it and hit details. This will show you all leveled lists it is used in.
Now you can track those down in Leveled list tab and open up one of them. On the right side of that leveled list window you'll see all the stuff that might appear from the list where that list is placed into a container. You can remove that specific weapon from the list now.
What I suggest is, create a new leveled list (by right clicking on the leveled item tab list and selecting new, or clicking edit on one of the references, changing the ID and saving it as a new object). Now find your weapon in the weapon tab, and drag it into the leveled item window (drag it specifically to the list area). Here on the list you can edit the number on the left of the name of the weapon, which will restrict the level that your character needs to be in order to spawn the weapon (so if you set it to 5 the weapon in the barrel won't have any chance to appear before your character reaches level 5), and on the left side of the window you will find "Chance none" option. This number is the % of which weapon will NOT appear. So if you set the chance none to 10% it means that there is 90% of chance that the weapon will appear from the list. If you set it to 90% then there will be 10% of chance that it will appear, thus making it much harder to appear, especially if you combine it with high level restriction, like 40 or so.
Now all you gotta do is find a container you want it to appear in (you can make your own too, if you want to, just rename the ID of some container and place it in the world), and drag the leveled list into the container's window, and voila!
Sorry if something is not understandable, if so let me know and I'll try to go more in detail
EDIT: what I am referring to as a leveled list, I meant leveled item