Monk's Deflect Arrow vs Wizard's Gravity Bow?

[Gruzzle]

Taking some relatively new players through their first full length campaign (Rise of the Runelords) and they've encountered an Npc with levels in monk, and the tiefling wizard is a transmutation specialist who loves her gravity bow spell on her heavy crossbow. In the moment, i wasn't sure which way to go with it, so i let him take half damage from her shot, he deflects it but it still tears up his forearm in the process due to its sudden density/weightiness. I made it clear to the player that I'd give a final ruling at next session though since this was just a compromise to keep things moving.

I got up today and went in search of answers and I have been unable to find any kind of thread about this. Is there a FAQ I've missed? Or an aspect of this interaction that makes it clear that I'm not understanding? As I read it, the spell doesn't make the ammo magical, per se. They're not "massive". What say y'all? Can a gravity bow shot be deflected?

[Eldariel]

By RAW, Gravity Bow merely modifies the attack. It's by no means as big as a ballista bolt (a light ballista deals 3d8 damage (http://paizo.com/pathfinderRPG/prd/ultimateCombat/combat/siegeEngines.html), equivalent to a Huge Heavy Crossbow - though even if it were, siege engines are different from handheld weapons by RAW so it wouldn't matter) and it's not a spell effect (this refers to things like Melf's Acid Arrow, which is explicitly a spell effect), nor a natural attack. Thus, perfectly deflectable. Though houseruling that the fluff of a surprising increase in weight surprises the Monk is reasonable if not RAW. Gravity Bow interacting with Deflect Arrows might be logical but power-wise, eh, Deflect Arrows needs all the help it can get already. RAW is clearly that they can normally be deflected; Gravity Bow influences nothing that the feat cares about.

[Necroticplague]

Deflect Arrow works perfectly fine on these enhanced bolts. I'm not seeing any reason it would be otherwise. The attack is generated by the bow, not the spell. This seems to me like asking if a Cat's Grace counters Deflect Arrows.

[Arkain]

I don't see anything in the spell's or the feat's description that implies that there's any interaction at all. Deflect arrows allows you to deflect once per round "when you would normally be hit with an attack from a ranged weapon". Key here is that it has to be a ranged weapon, like a bow, sling or whatever. A ray generated by a spell (thus not a "real" weapon) would go through. It doesn't matter if said ranged weapon is magical, either, the attack is deflected, period, probably even if it's a bolt from a gargantuan crossbow. You might decide that it's silly, of course, but that'd be mostly houseruling.

[Falontani]

By RAW, deflect arrows works perfectly normal

However the way you made it work is great; I would have the monk be able to deflect future shots from a gravity bow perfectly normally if they are expecting it. The first or even second time however should still come as a surprise to some creatures

[Gruzzle]

Right, thank you everyone! I guess i had just interpreted Deflect Arrow's text saying "spell effect" as being undeflectable as possibly including gravity bow, since it's an effect on the ammo, not the bow.

[Deophaun]

As a general balancing principle, if you're unsure if a mundane ability counters a magical ability, side with the mundane. Mundanes are much weaker generally and far less prone to break things.