What sin falls upon the deceased because he held his father's licensed weapon without his knowledge, and is his death considered martyrdom?

We ask Allah to have mercy on your brother. We hope that he incurs no sin for what happened if he intended to clean the pistol and was hit by a bullet he thought was not there, for the Prophet, peace and blessings be upon him, said: "Indeed, Allah has pardoned my Ummah for their mistakes, forgetfulness, and what they are forced to do."

Sheikh Muhammad ibn Ibrahim was asked about a man who accidentally killed himself: Is expiation or anything due from him? He replied: "Whoever accidentally kills himself owes no blood money or expiation, nor is anything of that due from any of his relatives," relying on the Hadith of Salamah ibn al-Akwa' concerning the story of 'Amir ibn al-Akwa', whose sword struck the back of his knee, and he died from it. The Prophet, peace and blessings be upon him, said: "Whoever said that has lied. Indeed, he will have two rewards." Scholars have unanimously agreed that delaying the explanation beyond the time of need is impermissible for the Prophet, peace and blessings be upon him, which indicates that nothing is obligatory.

As for his martyrdom, we do not know of anything that indicates it.