I just realized one more thing, which might have been a mistake on your part: You say you can change the radius of the AMF at will, with Artifactwork Cold Iron armor. This means you could change the radius to 0', and be permanently immune to all magic.
Instant conjurations go trough AMFs just fine.
Okay, most magic. It's still pretty powerful to be immune to almost every targetted or AoO spell, SLA and SU in the game, with no penalty to yourself at all (since you're not using magic items). In the normal game, Antimagic Field comes with serious down-sides.
It has big penalties to yourself. You can't benefit from your own Su abilities, receive buffs from party casters, receive su auras such as bardic music, etc, etc.
It's good to hear that at least two people around here think that they would still pick magic items over pure metal. That shows that it's not quite as unbalanced as I think, at least not in the types of games that two people play in (highly optimized games, I assume). Really, this probably just needs to be playtested. I imagine that I'm right about some of the balance problems, and wrong about others. In particular, it would be worth testing what happens when a party of PCs with magic equipment go up against a party of monsters with pure metal (and maybe one spellcaster, to balance things out).
Well, if you go by standard treasure, monsters will never have artifactwork or phantasmwork stuff (lv 20 average monster treasure is just 80 000 GP, relicwork alone has value over 115 000 GP). Only after level 13 they can afford relicwork, assuming they're medium or small sized.
And then, many monsters rely on powerful SLAs to support themselves. If they wear pure metal, they may gain raw combat numbers, but pay that in utility and variety. No teleports, mass charms, invisibilities, save-or-suck, etc, etc. And must still spend feats becoming proficient.
The monsters that at the top of my head benefit more would be giants, since they don't have SLAs but do have nice weapon and armor proficiencies.
Let's take the iconic fire giant, 5800 GP to spend. Make its half-plate pure iron for 2400 GP, then its greatsword pure cold iron for 1400 gp, and a back-up pure silver longbow for 1710 gp in case you have to face ethereal opponents or just need ranged combat.
So that gives it +11 to damage rolls, enemies can't cast defensively and ignores all miss chances when it threatens a crit (10% of the time), +9 to AC for a total of 32 (and a touch AC of 24, meaning touch attacks don't auto-hit it) and hardness 8 (nice because it still didn't have any DR).
It's stronger? Yes. Will it auto-crush parties of 10th level? No. If anything, it may actually give them a good challenge now, because he can actually harm ethereal opponents, casters can't just stand in front of him laughing while maiming him with touch attack, and it's also harder to just kite him with ranged attacks while flying.
Regarding AC, yes, AC is probably overpriced, especially on armor, but it's still an important factor. If you use standard, unmodified monsters from the Monster Manual, and your AC is 22 higher than expected, there's a good chance the monsters aren't going to be able to hit you, except on a 20. By providing such a huge AC bonus, you're forcing monsters to have better equipment or class levels, or common access to certain spells.
If there's something that MM monsters have no lack of, is massive to-hit bonus. A black wyrm has a whooping +46 to hit before you start factoring in feats, spells or equipment. Often the question is if the monster will hit, but how much you can power attack with before you start missing with anything but natural 1s.
On the PC, I will probably be building a character using pure metal in the next couple of days. It may or may not end up being played but there's a possibility.
On the DM side, I would not be at all surprised if Oslecamo already has encounters involving pure metal equipped monsters lined up in one or more of his games. This is the guy who wrote the handbook on souping up monsters to challenge PCs, after all.
Just planned? I already tried it out online.
I get your point, but the fact that the game has imbalances is not an excuse to add an additional imbalance.
If one side of the scale is too heavy, you add some weight to the other side. That's how balance works.
Add too much weight to both sides and the scale breaks.
Trust me, I would love to take some weight on the heavier side of the scales, but it's my experience that a massive fraction of the 3.X gaming population will cry foul if anyone dares to try to make any significant neerf on wizard/cleric and friends, at least not whitout giving said wizard/cleric even more buffs in return (see: 4e's fall and pathfinder's rise).