How do you unlock the Japanese Musket in MGS Peace Walker

You have to complete Extra Ops 121 or 122 (doesn't matter) and damage the wings of Rathalos ( I damaged all the parts like the tail and head just in case).
Dont kill Rathalos though, you have to tranquilize Rathalos after you have damaged his body parts. The Musket will then be unlocked once you have completed the mission. The design documents will say "Rathalos Design specs"
and when you go back to mother base you will have the musket ready for design.
DONT FORGET that you also need to have an R&D member to have Japanese Patriot skill to develope this weapon.