Green’s Dictionary of Slang

(US black) a .38 calibre gun.

1992		St. Louis Post-Dispatch (MO) 2 June 6A/3: Trey eight — .38-caliber gun.
1998		Mayor’s Anti-Gang Office City of Houston, TX Street Gang Sl. / Gloss. 9 Nov.: trey eight A .38 caliber handgun.
2005		R. Cea No Lights, No Sirens 37: This he screamed so there would be no miscommunication between him, my trey-8, or Billy [...] 245: This had to be a first for him, on the right end of a chewed-up trey-eight.