Hello this is my guide for ADC Kog'Maw. This one i want to be the best of my guides, i hope it will be so :)

For Runes i recommend Armor Penetration Quintessences, Attack Damage Marks, Armor Seals and Scaling Magic Resist Glyphs.

For Masteries i recommend 21/9/0 (There is not much to say about the Masteries, just don't take the 2 points in Lethality and 1 in Frenzy when building the Glorius Bombardier or Rocket Launcher)

For Classic Kog'Maw i don't recommend to buy both Bloodthirster and BOTRK because of the great critical Damage you don't need much Life Steal % for big heals.

The Skill Sequences are:
Build 1 (Classic ADC): E-W-Q, R is used when chasing or checking bushes (Living Artillery reveals enemies)
Build 2 (The Glorious Bombardier): There is no combo for this build, all you have to do is staying out the range of your enemy and spamming your ultimate, If fighting somebody 1v1, use Q to reduce his/her armor/magic resist W is your main Ability, activate it immediately, use E when your opponent will try to escape, or when you will try to escape.
Build 3 (Rocket Launcher): Here all you have to do is spamming the ultimate.

I recommend the standard ADC Spells (Flash+Barrier) or if u like Cleanse take Cleanse+Flash.
Ghost is also usefull when chasing somebody to kill him with your ultimate.

Pros:
-Awesome Range With W and R
-Big both Magic and Physical damage on attacks
-Passive that sometimes lets u get some kills
-Good slow
-Easy farming and lasthitting
Cons:
-Easy to counter the Passive
-Useless without farm and kills
-VERY Squishy early game
-Q is actually useless on adc (so its like you would have 3 abilities)
-The ult cannot be used to often, it costs to much mana at 5 casts already

You can always help your allies with your E that has a great AoE slow. Your ultimate is also very helpful when running, it may turn the tables, and make the prey to the hunter.