Kog'Maw Runes explained
There's one set of runes for Kog'Maw on patch 14.23 that is the clear favorite for LoL players and pros alike. The best Kog'Maw rune paths on the position 
Adc are 
Sorcery as the primary path and 
Precision as the secondary path with a pick rate of 77%. Besides picking runes on Kog'Maw you should learn how to play Kog'Maw and if you are interested in pro matches take a look on how a pro builds Kog'Maw. If you want to discuss 
Kog'Maw then you should visit the champion subreddit.
Kog'Maw runes from primary path
Sorcery empoweres abilities of Kog'Maw and increases the available champion resources. Most of high elo players – and our LoL pros in the the major regions – have agreed that Arcane Comet is the best keystone rune to run on Kog'Maw. This keystone rune adds more damage to your abilities and increases your power in the laning phase. Other primary runes on Kog'Maw are Manaflow Band, Absolute Focus and Scorch.
This is usually followed up by taking Manaflow Band for additional mana and mana regenaration. Absolute Focus grants additional damage while Kog'Maw is alomst full health. Scorch increases your control in the early game.
Kog'Maw runes from secondary path
When it comes to your secondary path, most Kog'Maw players prefer to go with Precision. Precision improves attack and sustained damage on Kog'Maw troughout the match. On the Precision path, the best runes for KogMaw are 
Presence of Mind and 
Cut Down.
Presence of Mind helps Kog'Maw with the mana problems by restoring missing mana, if a champion is killed. If the enemy team consists of multiple tanks, then the rune Cut Down is the best choice.
FAQ for Kog'Maw Runes
Which is the best keystone rune on Kog'Maw?
The best keystone rune on Kog'Maw is Arcane Comet. This Kog'Maw runes setup is then referred as Kog'Maw Arcane Comet.
What runes does Kog'Maw use?
Runes from the paths Sorcery and Precision are the most used runes on Kog'Maw. For the exact pick rates of Kog'Maw runes used, you have to study the rune distribution presented.