747.1. Weaponmaster is a Triggered Ability keyword.
747.1.a. Weaponmaster is present on Units.
747.1.b. Weaponmaster is a Play Effect that chooses an Equipment you control and allows you to pay its Equip cost at a discount, regardless of the usual timing of the Equip ability, to Attach that Equipment to the unit with Weaponmaster.
747.1.c. Weaponmaster is functionally short for: “When you play me, choose a Card you control with the Equipment tag. Necessary portions of its Rules Text are no longer Inactive if they are currently Inactive. Pay the cost of its Equip ability, reduced by [A], to attach it to this unit.” See rule 716 Attachment for more information.
747.1.c.1. Weaponmaster can choose an Equipment whether it has an Equip ability or not.
747.1.c.2. The cost of the Equip ability is determined as though that Equip ability was being activated choosing the unit with the Weaponmaster ability, as modulated by any abilities that alter Equip costs.
747.1.c.3. If the chosen card’s Equip cost does not contain [A], it can still be paid, but will not be reduced.
747.1.c.4. If the chosen card doesn’t have an Equip cost, it can’t be paid.
747.1.c.5. If the chosen card’s Equip cost can’t be paid, if it can’t be detached from its current Top-Most card, or if it can’t be attached to the unit with the Weaponmaster ability, it stays in its current location, Attached to anything it was already Attached to.
747.1.c.6. The Equip ability is not activated this way, and the unit with the Weaponmaster ability is not chosen.