Heya! First guide here, so I'll try to keep it concise. This is my build for when I want to hop into the fray, spam missiles at anything that moves (hence the name - with the Butcher skin, it might as well be raining chainsaws), and swap fleeing champs into the maw of my teammates. So, without further ado...

9x Greater Mark of Desolation - pretty self-explanatory; all of Urg's attacks are physical, and these make his missiles that much more awesome.

9x Greater Seal of Clarity - With all that missile spam, Urg runs into mana trouble early and sometimes mid-game; this helps him stay in the fight, popping shields, swapping fools and spamming missiles like a boss.

9x Greater Glyph of Focus - Cuts the already low CD of his missiles even more, and by late game he's launching one almost every second, stopping only to drop charges on peoples' heads and keep his shields up.

0/9/21 - Gives the Big Urg some boosted health and mana regen, increased resilience, a bit more mana, and all that other tasty goodness a mana-thirsty cyborg could need.

Start with a Meki pendant and a couple of health pots; this will help alleviate Urg's mana issues at the start, and give you some staying power early on. When you get 955 gold, warp back and buy the speed boots and the tear of the goddess; the boots will grant enhanced mobility while the tear will go further to help Urgot's mana. Then, at your discretion, buy mercury treads, the manamune, and the brutalizer; this will give you a good core of increased mana, increased damage, even more armor pen, and will drop your CDs lower. Then, follow up with Aegis of the Legion (for extra resilience, as well as the team boost) and a glacial shroud, further increasing your mana (and ergo damage), an frozen heart (even more mana/damage CDR and armor).
If the other team is still standing, follow up with the BF sword, then the Black Cleaver (as if you weren't making enough of a mockery of their armor), and Youmuu's Ghostblade to tie it all off, if you have time/spare change.

Missiles, missiles, and more missiles! You max out Acid Hunter, followed by your shield (for extra resilience and slowing power), with a point in the corrosive charge at 2 so you can get missile lock. Points in the R at 6, 11, and 16 as normal.

Flash/Exhaust - Flash helps the Urg get out of a bind that his shield and armor can't readily ard off, while Exhaust can help set up a missile lock/death rain barrage, or set up a position swap into your teammates or a turret.

Mucho props to Searz and his Crabgot guide for inspiring a lot of this (part of the build order, skill sequence, and the mastery setup). Now...BRING THE RAIN!