Any attempt to will just result in the gun dropping to the ground as if your inventory is full.
Note that you cannot pick up any firearm you don't have the licence for. The Rifle Licence requires the defeat of Charles, a rifle wielding contender in the tournament when representing the Disciples of Prajna on at least Hard difficulty. The Gun Licence unlocks for purchase with Samurai Points once you defeat J.J. A Gun Licence allows the use of pistols and a Rifle Licence allows the use of rifles. To use any firearm in the game, you have to get the proper licences. Appraising a firearm with the "Unbreakable" moniker will negate the effect firing shots will have on the durability of the weapon, essentially making a gun that never breaks, making the user almost invincible since they never have to get close to any opponents and just fire from afar. Firearms are usually not that powerful unless some heavy upgrading takes place. It is recommended to bring and apply blade oil to the firearm before going into a fight and reapply when possible. This also causes firearms to break often as each round fired takes a significant chunk of the durability.
When fired, they will decrease the durability of the weapon each time, regardless whether the shot hits or not. Firearms work differently than melee weapons in the game. They both function almost identically but with differing styles. There are two types of firearms in the game: Pistols and Rifles. Firearms are the only ranged weapons in Way of the Samurai 4.