I don't think that assault of a police officer is an automatic Class A felony.
Here's what I found in the Missouri statutes:
565.082. 1.
A person commits the crime of assault of a law enforcement officer, corrections officer, emergency personnel, highway worker in a construction zone or work zone, utility worker, cable worker, or probation and parole officer
in the second degree if such person:
(1)
Knowingly causes or attempts to cause physical injury to a law enforcement officer, corrections officer, emergency personnel, highway worker in a construction zone or work zone, utility worker, cable worker, or probation and parole officer
by means of a deadly weapon or dangerous instrument;
(2)
Knowingly causes or attempts to cause physical injury to a law enforcement officer, corrections officer, emergency personnel, highway worker in a construction zone or work zone, utility worker, cable worker, or probation and parole officer
by means other than a deadly weapon or dangerous instrument;
Assault of a law enforcement officer, corrections officer, emergency personnel, highway worker in a construction zone or work zone, utility worker, cable worker, or probation and parole officer
in the second degree is a class B felony unless committed pursuant to subdivision (2), (5), (6), or (7) of subsection 1 of this section
in which case it is a class C felony. For any violation of subdivision (1), (3), or (4) of subsection 1 of this section, the defendant must serve mandatory jail time as part of his or her sentence.
http://www.moga.mo.gov/statutes/c500-599/5650000082.htm
My understanding of the above statutes: MB punching Officer Wilson was a Class C felony.
MB's struggle with Officer Wilson over his service weapon may have been a Class B felony, since the firearm discharged during the struggle.
Here's the Missouri statute regarding 1st degree assault of a police officer:
565.081. 1.
A person commits the crime of assault of a law enforcement officer, corrections officer, emergency personnel, highway worker in a construction zone or work zone, utility worker, cable worker, or probation and parole officer
in the first degree if such person attempts to kill or knowingly causes or attempts to cause serious physical injury to a law enforcement officer, corrections officer, emergency personnel, highway worker in a construction zone or work zone, utility worker, cable worker, or probation and parole officer.
7.
Assault of a law enforcement officer, corrections officer, emergency personnel, highway worker in a construction zone or work zone, utility worker, cable worker, or probation and parole officer
in the first degree is a class A felony.
http://www.moga.mo.gov/statutes/c500-599/5650000081.htm
Definition of "serious physical injury":
(6) "Serious physical injury" means physical injury that creates a substantial risk of death or that causes serious disfigurement or protracted loss or impairment of the function of any part of the body
http://www.moga.mo.gov/statutes/C500-599/5650000002.HTM