Explain AC vs SR to me

[weckar]

I have a problem with SR. Earlier, I've had it explained to me like "AC, but for magic".

My problem lies in the fact that overcoming SR seems to be a matter of training only. What you use, or any natural ability, don't factor in.

To make a comparison:

Overcoming AC: 1d20 + BAB + STR/DEX + Weapon Bonuses
Overcoming SR: 1d20 + Caster level (...)

Basically, I'd expect the primary casting stat and maybe even spell level to also factor in. Stronger spells would more easily overcome, intruitively.

Can anyone explain to me why SR DOES NOT work like this?

[Zanos]

Fair warning, this is a bit of an oversimplification. Magic is often considered too versatile in that spells exist that don't require any rolls for their effects to take hold and there's no means of resisting.

Because SR is not AC for spells. The primary defense that spells target are saving throws, which Spell Level and Casting Stat do affect. Many monsters don't even have SR. They all have saves. SR is an alternative defense, like concealment. If you're having issues with it, there are ways to overcome it fairly easily. Assay Spell Resistance, (Greater) Spell Pentetration, Arcane Mastery, and general caster level increases. The original intent of SR is to provide a feat tax for casters so that they can be routinely effective against certain creatures.

[Troacctid]

It comes directly from how magic resistance used to work in previous editions. It was a percentage chance that magic didn't work against you. If you had magic resistance 50%, it meant there was a 50% chance that a spell cast against you would fail. However, higher-level casters could overcome it more easily: for each level above 10th, your effective magic resistance against them was reduced by 5%, and for each level below 10th, it was reduced by 5%.

3e essentially brought back the same system, but got rid of the subtraction to make the math easier. (See also THAC0.)

[Eldariel]

In addition to what Troacctid said, it's also a factor of math. Because you don't get any stat to Spell Resistance itself, the parity in the system would be broken if you got a stat to spell penetration. It would indeed be possible to give e.g. Cha to Spell Resistance by default and then give casting stat to Spell Penetration but that would further weaken Spell Resistance since few characters can afford Charisma high enough to match the highest casting stat of your average enemy. Then again, the game already has Assay Resistance so meh.

[weckar]

Guess I really just misinterpreted what SR meant...

[KillianHawkeye]

Guess I really just misinterpreted what SR meant...

To be fair, while explaining SR as being "like AC, but for magic" is a bit overly simplistic, it isn't a bad starting point for explaining how it works. You just need to keep going with how SR is similar to AC and, more importantly, how it is different.

[Der_DWSage]

I'm pretty sure that the 'AC, but for magic' explanation came more from the mechanics of what happens when it fails more than anything else. If you don't roll above a target's AC, your attack does absolutely nothing. Similarly, if you don't roll above a target's SR, your spell does absolutely nothing to them.

I can't really explain why spell level, casting modifier, and everything else doesn't factor in for you, because that would require prodding the developers' brains-but in most cases against an opponent with equal-level CR, the math still works out to a 50/50 chance to successfully land a spell without any feat investment.