You can have your legendary bone carver carve out of bone some bone gauntlets, which have protective value and weight (500 density?) like leather too. Also as part of leather armor, you can fit metal chain mail, which is part of metal armor. So long it is not your weaklings' crossbow"hammer"dwarves police squad, lead by your fortress's equally weak captain, then military dwarves (normal & strong) should have no issue lifting 20 units of chain mail weight under leather armor too.
Combat/Training bug is still on. You don't assign combat ammo in the same time with training ammo. If you do, then dwarves will put 25 bolts of one sort and then 25 bolts of another sort, then spam you during training "equipment assignment failure". In combat they will also spam you with "equipment assignment failure" and empty 5 training bolts futilely at enemy, before closing to close combat range and using their crossbows to hammer repeatedly enemy.
1. Assign only training (wooden & bone) ammo to hunters and military squads.
2. When your hunters reach legendary levels, assign them metal bolts. It is ok then, specially if you keep smelting all recovered single metal bolts.
3. When you want your crossbowdwarves to change for combat ammo, then you need to
- in archery disable training.
- in military menu remove training ammo and add combat ammo.
- in u-units menu, find all crossbowdwarves and mark all training ammo in their quivers for dumping.
- station the crossbowdwarves
- make few checks on them all, if their training ammo is dumped yet and if military ammo is loaded yet.
- if the checks are go, then you can use them for combat, sentry assignment or hunting dangerous savage animals, which in a large group just entered your map.
I know, it is a lot of tedious micro management, but like forcefully hammering too big bolt into too small hole, you can make it work... eventually.
To reverse the change.
- in military menu change combat -> training ammo.
- in u-units find all xbow dwarves and mark in all quivers combat ammo for dumping.
- keep every while or so checking if they have dumped metal ammo yet.
- if check is ok, then enable archery training for the squad.