I found some new data that Nobunaga was not the only one who mastered the gun. Uesugi Kenshin as well knew how to use gun efficiently. I have to admit, I did not know Kenshin mastered the weapon. You have to give him his props.
Thomas Conlan's Weapons and Fighting Techniques of the Samurai Warrior explains on page 165.
"The example of the Uesugi suggests again that those most cognizant of tactical organization, possessed a very real advantage on the battlefield, and this explains why certain families survived and others were destroyed."
Conlan mentioned that Kenshin used guns and arrows so well that the Takeda suffered massive losses at the 4th Battle of Kawanakajima in 1561. The Takeda, on the other hand, were late to the party. I have two reasons why. Geography and Culturally.
The Takeda were at a major disadvantage geographically since they were locked in the mountains. To obtain high quality gunpowder and guns would have been difficult. Kenshin knew people who can supply him with the best and took full advantage of it.
Culturally, I think the Takeda had too much success using weapons that they were familiar with. I do not think the Takeda adapted quick enough to stay in power forever. Nobunaga and Kenshin knew the gun's potential to change warfare and quickly took advantage of it.
I am not here to rip on Takeda Shingen. I think he was a great warrior. However, I think he failed to know how to use the gun when needed. He barely made out alive at the 4th Battle of Kawanakajima and his son Katsuyori, failed to recognize the gun at its best. Nagashino anybody?
Tenka no Tame!