(May not actually be brass, but it looks like it)
From SonoKong Dranzer S, possibly other SonoKong releases.
Weight: 0.245g
From Bearing Gyros Series, Dranzer F, Driger F, Burning Kerberous and Zeus
Weight: 0.243g
Generic MR52ZZ Shielded Metal Ball Bearing (manufactured by NSK in early releases, manufacturer not named later)
Also called Free Bearing, NSK Bearing
From Metal Dragoon Bearing Stinger, Bearing Gyros Series, Wolborg, Wolborg 2, and Burning Kerberous
Weight: 0.200g