Did a quick search: In the Reimeikan in Kagoshima there is a history of gun production in Satsuma-han dating from 1601 written by Nanpō Bunshi, and it apparently has details of gun manufacturing in the 16th century. I think this is the primary source for much of our information on Portuguese weapons. The article below says that after buying 2 guns from the Portuguese, Tanegashima Toki was instructed to learn how to produce the weapons, and he succeeded in producing gunpowder and rifle barrels, but failed to produce the necessary screws. It took another year and another foreign ship to transmit the method for manufacturing screws, but in 1544 they were able to produce a few tens of guns. From there, the center of gun production moved to the Kinki area, and by the time the forces of Ishiyama Honganji battled Oda Nobunaga in 1570, they employed 8000 guns.
Anyway, this history of gun production might have the details you want. The above was just taken from some random gun enthusiast's site in Japan.
I guess the question of brass manufacture is really a question of zinc mining and/or importation, and the lack of a significant zinc supply in Japan seems to have retarded the Japanese development of brass. Wikipedia and other sources say that brass production (亜鉛, or 真鍮) by the Japanese followed their encounter with the Portuguese, although there is one article that notes the presence of zinc was discovered in the gold ink of a 12th century buddhist sutra scroll.
Other than this (apparently) minor amount, zinc seems to have been more or less overlooked/unknown in both China and Japan, until its production in China in the 16th century, after being imported from India.
Anyway, if you are looking for primary sources on gun production, I think the Reimeikan is one source.