Chainmail is a common medieval protection for fighters and adventurers. However, according to your campaign's TL, a chainmail may be more or less efficient in terms of protection.

* TL1: chainmail didn't exist yet at TL1.

* TL2: chainmail was "ring" mail, with interleaved metal rings. Those ring were often loose enough to be uneffective against piercing weapons (arrows, spears, etc.) and thrusting attacks made with small enough blades.

* TL3: technology regarding chainmail is evolving. Chainmail that is efficient against piercing/thrusting attacks is developed, but, according to wealth and supply, not everybody can get it.

* TL4: by TL4, most chainmail is efficient against any type of attack or weapon.

Note that weapons, at early TLs, were also quite large, and most of the time blunt because of their rough crafting. You may, therefore, state that, TL2 chainmail is fully effective against TL2 weapons, TL3 chainmail is fully effective against TL3 weapons, and so on. In this case, you would only apply penalties to chainmail whenever a lower TL chainmail gets hit by a higher TL weapon.

In terms of game stats, add another split DR to chainmails, that give reduced DR (or none at all!) against piercing/thrusting attacks, but only against weapons of higher TL. Also note that not all piercing weapons will be small enough to take advantage of this - a TL4 pike point is wider than a TL3 ring of chain mail. In game terms, perhaps only apply the effect to knives, fencing weapons, and possibly swords.