One of the reasons why ArPen runes went out of fashion was because of the 10% ArPen mastery, which made ArPen runes less efficent. On the flip side, AP carries have always had that mastery. It was actually better before, when it gave 15% Spell Pen.

Of course, it has always been the case that the Pen runes would not do well past the midgame because AD carries would get last whisperer, while AP carries would get Void staff, making the Pen runes not as beneficial.

So why did AD carries used to still get some ArPen on their rune pages? Because there wasn’t a cheap early-mid item that offered them flat ArPen. Tanky DPS champs had Brutalizer and Ghostblade as a good option, but AD carries for the most part went the standard bloodthirster or IE, zerker boots, PD, LW, which would last into late game.

AP Carries on the other hand would almost always pick up sorc boots, which offer a flat 25 spell pen. Factored in with the spell pen reds, unless the enemy champ specifically builds MR, abilities should deal true damage to squishies in the early-mid game.

The end result? APs don’t benefit as much as ADs when it comes to using flat Spell Pen in the early game. Honestly, the only reason AP carries still use Spell Pen reds is because the alternative AP red is really inefficient.

Reds: 0.59AP, 0.95 Spell Pen. Conversion 1AP:1.61 Spell Pen

Quints: 4.95 AP, 1.89 Spell Pen. Conversion 1AP:0.38 Spell Pen.

So looking at the ratios, AP quints doesn’t sacrifice a lot of Spell Pen, while AP reds would cost a lot more Spell pen. The quints are actually 4 times more efficient on AP/Spell Pen. If the reds converted on AP/Spell Pen in the same ratio as the quints, I honestly would probably remove spell pen for AP.

As it stands right now, the most efficient Quints/Red setup for AP carries is what everyone uses already: AP Quints, Spell Pen Reds.