The actor and his attorney Alex Spiro filed new legal docs, obtained by TMZ, where he seeks to have his criminal case completely tossed ... arguing the grand jury process was completely rigged by the Santa Fe County D.A.'s office -- alleging they trotted out 7 witnesses who were incredibly biased.

Specifically, Alec says 3 of the witnesses were on the D.A.'s "payroll," 2 of them were from the Santa Fe Sheriff's Office, one was already suing him in a separate civil case -- and yet another publicly blamed Baldwin for Halyna Hutchins' death days afterward ... even though Alec says this person wasn't working on the production at the time.

Another eye-popping allegation in the docs involves the FBI testing of the pistol used in the scene. Alec claims the D.A. failed to present all the evidence from that testing ... which he alleges showed the revolver did, in fact, fire without the trigger being pulled when all 6 rounds were loaded, just as they were on the day of the tragedy.

Remember, up until now ... we'd heard the opposite -- that the FBI testing proved without a doubt the gun could only fire if the trigger was pulled. This is crucial, because Alec has always maintained he did not pull the trigger.

For all these reasons, Alec's asking a judge to drop the involuntary manslaughter charge. As we reported, he already pled not guilty in this second go-around ... this after the D.A. dismissed their charges the first time.