Rapper Tone Loc pled "not guilty" today to two felonies in his domestic violence case -- 1) beating up his baby mama ... 2) possession of an unregistered, military-grade ASSAULT RIFLE.



According to police, cops found the unregistered Colt AR-15 Sporter during an investigation ... after the rapper was arrested on suspicion of domestic violence last month.



As we previously reported, Loc -- real name Anthony Smith -- was busted in a Burbank, CA apartment complex on June 18 after an alleged physical altercation with the mother of his child.

Loc is due back in court in two weeks.