... is easy. Getting it upgraded to Level 3 is a different matter.
Because, you see, that requires metal. Okay, that’s what a dispenser is for, right? Of course of having a dispenser to produce metal you have to build one first. And if you want the dispenser to give you metal in more than a leisurely pace that dispenser needs upgrading, too. Which again requires metal.
Well, usually there’s a lot of metal lying around in different sized packages. So all is simple enough then ...
In theory, yes.
Unless you are part of the Brainless Crew™.
You know, the one that spends its time during setup wildly shooting their weapons for no other reason than that they are BORED (besides brainless), taking target practice on inanimate objects (hey, l33t skills need such sophisticated practice) or rocket/sticky jump all over the place (and no, often NOT to help a medic build a uber).
And naturally after wasting their precious ammo they have nothing better to do than to run and take the next ammo pack to fill up their stock.
Which is - of course - exactly the ammo the engineer would need to build and upgrade his buildings!!! Morons!
Nothing like having a sniper firing 2 shots just to pick up a large ammo crate which would have given the engy 200 metal (= one full upgrade for a building).
All this is particularly annoying on spots which have no large ammo crates, like Gravel Pit, Badlands or Granary. Getting one’s buildings up here is already more of an artform which involves careful planning without having it sabotaged by braindead teammates wasting the ammo without distress.
Remember, once the dispenser is set up you’ll have plenty of ammo at your disposal. And please, if there’s a dispenser nearby, use that instead of further picking up ammo crates. The engy might still require those to upgrade his buildings (which works much faster than waiting to draw 200 metal from a dispenser).
Also keep in mind that when playing defense on maps like Gravel Pit or Goldrush you may be bored waiting for the gates to open, but the engy is already working very hard to get his stuff ready enough that it may withstand the first rush of attackers.
So next time the engy on your team is running collecting metal around like a squirrel keep in mind: HANDS OFF THE METAL unless you really, really need it.