According to the Wiki, a single item produced gives 30 experience in that skill (with some exceptions). 400 items is 12000 xp, which should be enough to increase a skill from "not" to Great (and almost to Master), or from Skilled to High Master.
So, the questions are... how many things has he actually produced? (You mention metal bars - I'm assuming you have a Master Furnace Operator!?) And is he the only armorer active?
But yeah, if nothing is going wrong, two years of repeat should have produced something near Legendary.
Dear Armok it feels like my Armorer/Weaponsmith has not even gained a level. I have 400 copper bars and just set my magma forge to repeat copper bucklers...
You may have made a conscious decision for other reasons, but for at least two reasons bucklers are the wrong item to practice on. Unless the situation dictates otherwise (many archers, mandates, whatever), make
greaves, or at least shields.
For the same single bar, bucklers have a base value of 10, shields 15. So even if you're melting them back down, your fortress wealth is 50% higher for those waiting to be melted. Greaves have a base value of 30, 3x the value of bucklers. If you have only 30 *copper bucklers* lying around waiting to be recycled, that's almost 5000 db's of value you're throwing away - no bankbreaker, but important to some, esp early on.
But more importantly, when melting, diff items produce a diff return of metal. This is based on their "material size" (
http://www.dwarffortresswiki.net/index.php/Melt). You'll always lose some metal, but there are some items that are better or worse in each skill-training category. For armorsmithing, bucklers return 20%, while shields return 40%. Greaves return 60% for the same single bar consumed in construction (the max possible). (Remember that these fractions are saved at individual smelters, not combined as a whole across your entire fortress.)
So, if you make 400 bucklers, and melt them all down, you'll get 80 bars back. If you make 400 greaves, you'll get 240 bars back. Your call.