Can you react to Weaponmaster trigger?

Yes, you can react to a Weaponmaster trigger.

According to Rule 747.1, Weaponmaster is a Triggered Ability. When you play a unit with Weaponmaster, the ability triggers and is placed on the chain as a pending item (Rule 376.3). Because it uses the chain, players are permitted to respond to it with reactions.

How it works:

1. Playing the Unit: When you play a unit with Weaponmaster, the unit enters play. You cannot react to the unit being played itself.
2. Triggering: The Weaponmaster ability triggers and is added to the chain.
3. Reaction Window: Once the trigger is on the chain, the state becomes closed (if it wasn't already), and players may add reactions to the chain in response to the Weaponmaster trigger.
4. Resolution: If no further reactions are added, the Weaponmaster trigger resolves, allowing you to choose an equipment and pay its cost to attach it to the unit.

This is consistent with the general rule that triggered abilities use the chain and can be reacted to (Rule 376.3).