Just the regular ones, not steel. If you look, you'll see that regular bolts have the following stats:
weapon length: 63
weight: 2.5
weapon length: 63
max ammo: 16
thrust damage: 0 pierce
slots: 1
As opposed to masterwork bolts, which are:
weapon length: 63
weight: 2.5
weapon length: 63
max ammo: 16
thrust damage: 0 pierce
slots: 1
I think this is probably unintentional, and would suggest improving the damage to +1, +3, +5 at each applicable heirloom level. Or just updating the website if this is already the case.