Making helmets have an in-game purpose...

[Oryan77]

Would there be any big downside to having helmets give either a Deflection bonus or some sort of DR for a campaign that doesn't allow PCs to buy magic items from shops?

[ValhallaGH]

Big downside? Not unless something there's something you've not mentioned.

What they do? That depends upon why you want to give game mechanics to helmets, and how attractive you want those mechanics to be.

Good luck.

[Celebrim]

I think helmets only really make sense if you are using 'called shot' rules. Called shot rules really don't make alot of sense in D&D because they make the game much more lethal at high levels, and regardless of the game I find that they add so much complexity that even a cinematic combat leaning person like myself finds the idea burdensome to track.

One option might be to divorce fortification from armor and make it a component of helmets, taking care to balance this option (as powerful armor and powerful fortification would now be cheaper if you didn't make changes). Under this model, even a non-magical helmet might add to your fortification at the cost of some penalty to spot and listen. Of the top of my head.

Metal Cap: 5% fortification, +5% chance of spell failure
Helm: 15% fortification, +10% chance of spell failure, -2 to spot and listen.
Full Helm: 25% fortifaction, +15% chance of spell failure, -4 to spot and listen

Magical Helm: +5% fortification per +1 bonus, maximum +5. Cost as per armor.

[Stringburka]

These are my rules for helmets:
Rules for helmets!

[wotmaniac]

I like what the previous 2 posters have said. to add to that:
way back in 2nd.ed. Dark Sun, they had rules for "piece-mail armor". basically, your AC was determined by how many parts, or "pieces" of armor you had. E.g., each leg guard gave you "x", each arm guard gave you "y", a helmet gave you "z", etc. (the flavor for this was that, given the scarcity of resources on Athas, you simply went with whatever it was you could scavenge/salvage)

[Ashtagon]

The standard 3e rule on helmets is that they are an integral component of the various "suits" of armour. In other words, you get an appropriately styled helmet with full plate, and if for some reason you decide to go hatless, you'd take an AC penalty.

I like the idea of helmets providing fortification bonuses. I would make one small proviso: the value of the (non-magical) fortification bonus should be capped by the type of armour you are wearing (max 25% for heavy, 15% for medium, 5% light, or something like that). Someone who is naked but for a great helm shouldn't really be getting fortification bonuses.

[Chronologist]

I agree with Ashagon, helmets are usually part armor, not having them reduces armor. For wearing a helmet with armor that usually doesn't have a helmet, I'd personally go with a very simple +1 bonus to AC, but a -4 penalty to Spot and Listen.

[Wulf Ratbane]

I hopped into this thread to suggest fortification and pretty much agree with what Celebrim said.

I especially agree that fortification could be divorced from armor and given over entirely to helms.

I think 5% is too low for the baseline, however. I'd not go below 10% in order to justify the time it takes to roll.