That's the thing, though. You can't just shoot through solid metal with primitive guns. The term "bulletproof" comes from the practise of firing bullets at new pieces of armour to make sure that they could withstand the blast. This is what's called "proofing" the armour.

Besides, it's fantasy. If you say skilled fighters can dodge bullets, they can dodge bullets. Heck, even if you don't allow guns, what's to keep us from simply using crossbows? Later winded models would penetrate most armour.

And besides, if we've got magic, what's a gun compared to a guy who can just lob fire at you? No armour's gonna stop a gout of fire, if you insist on being realistic.