Bowcraft training method.
Go to Heartwood.
Do the "A simple Bow" quest (bows) until 60*
Do the "Ingenious Archery I" (crossbows) until 80-85*
Do the "Ingenious Archery II" (heavy crossbows) until 90-95*
Do the "Ingenious Archery III" (repeating crossbows) to GM.
* The uncertainty is the fact that the sweet spot for crafting gains is between 60% & 80%, but the above examples all go over 80% before the next item to craft/train hits 60%. So, the transition point is up to the crafter.
Gains are possible until 70 on bows, to GM on all the crossbow types, though for diminishing returns above 80% chance of success (see the bowcraft/fletching calculator for exact numbers)