For each type of weapom you will need to place it attached in the correct position, then you will need to take notes of the complete transformation stats (position, rotation & scale (not usual)) place those values in a data table and when you have to equip that type of mesh you will copy those values from the table and apply to the mesh. So, this data table will have the mesh and transform info for each type of weapon. To get this more advanced, you can have this data for when it is equipped and unequipped, if there is no entry for unequipped you just make the mesh invisible, if there is an entry you can attach it to a different part of the body, like the back or the side of the character.