Well, here it is. My first build post. This build is for poking, very early and some what late game dominance. People who don't enjoy risking their necks for gains, this build is tough to play. Since most of Rumble's attacks are close ranged, it's very dangerous. Proceed with caution...

Now, with the runes, I was pretty set forward on getting the early game dominance. With Rumble's spray and focus fire magic abilities, early game magic penetration can count for so much, especially in the start when people usually have only 20-30 some armor. If you really don't want to go with weak ability power seals, then try getting dodge seals or armor seals (such as Greater Seal of Evasion or Greater Seal of Defense). I do not recommend it, though.

With masteries, I went all out 'n out for ability power, cool down reduction, and increased damage output (the masteries which increase all damage by percentages). I really don't see any other way to build, except maybe going a little more support for the extra cash or cool down reduction, but then you wouldn't get enough in offensive as this build is made upon.

Items, they can make or break a game. To start off a game I would usually get boots, then 3 health potions. Though the boots don't really give much benefit except for getting to your lane as fast as possible, the 3 health potions help you hold the lane for a much lengthier time. What I usually try to build up first, after buying the boots though, is lich bane. Yes, I know what you're thinking. "Oh, Rumble doesn't have mana, why should I get it?". Well here's why: It's passive. When Rumble stacks lich bane's additional melee damage for a hit (coming from your AP count), Rumble's magic damage from over heat, and the last spell Rumble cast, when you go in for a hit, that (at full build) could be around 350 melee damage, plus about 200 magic damage. That's over 500 damage in one hit, without counting it could crit, they could have negative armor from magic penetration or some such spell, them taking increased damage from something, you never know. After lich bane, build sorcerer's shoes. Once that's built, build however you like.

P.S. Elixir of Brilliance is a great item in the game at anytime. It gives a nice chunk of ability power and a huge cool down reduction. Great for if you know there will be a team fight soon, so you can pump your flamespitter.

Though this doesn't matter much, I recommend getting level 1 electro-harpoon at minimum, then try to max flamespitter as fast as possible. It has great aoe, and damage. This helps in feeding, team fights, or just plain 1v1. (HINT: LEVEL ULTI AS SOON AS POSSIBLE! IT IS RIDICULOUSLY EASY TO GET AN ENEMY HERO RUN-AWAY WITH!)

Get surge and ignite for one reason and one reason only: ability power. With summoner's wrath selected in the masteries branch, both help increase attack damage and ability power, while also doing damage over time (ignite) and increasing attack speed (surge). Besides that, I used to use exhaust and ignite, since exhaust (with the armor and magic resist decrease from the mastery tree) helped slow and weaken the opponent, making them/it fodder for you or your team mates.

Pros:

Easy to run in and dominate in 1v1 or poking (dealing constant, though maybe light, damage)
Burst damage helps farming
Pretty nice in team fights
Great for catching run away kills with scrap shield and the equalizer
Does both heavy melee and magic damage, due to lich bane's passive and abyssal scepter's magic resistance reduction and sorcerer's shoes magic penetration

Cons:

Somewhat squishy
If silenced, Rumble becomes close to useless
No good range except for ulti, so easy to pick on by ranged