Fridge Logic: Where in the world did he find (and how did he legally obtain) ammunition for the main gun?
He is a senior NCO in an armoured regiment, and ammunition for the M4 Sherman's 75mm cannon was produced in such huge quantities that the US Army was probably finding long-forgotten crates of it kicking around in storage for years after it was made obsolete. Which of course raises some more Fridge Logic: How did he smuggle it out of the base munitions store and into his garage?
The Sherman was still in active service in many smaller armies until the late '70s, and the last three in service anywhere were only retired in 2018, so the ammo wouldn't be too terribly hard to come by in the early '80s. Additionally there's at least one American company that lets people pay to drive around their restored Sherman and even fire live ammunition from the main cannon. This may be a case of Reality Is Unrealistic.
Fridge Horror: Why did he smuggle it out of the base munitions store and into his garage?
Hearing G.D. Spradlin say the words "Pussy Communist" is worth the price of this movie.
Shirley Jones, as LaDonna Carey, has this moment, which counts as atripletrope:
LaDonna Carey: Well, my perspective has about a dozen reporters from every TV and radio station and newspaper you can think of, waiting for me to come outside and make a statement. Now, I'm either going to tell them that they have a governor that they can be proud of, who's willing to see that justice is done, no more, no less. Or, I'm going to tell them that their governor is an asshole!
Gov. Harold R. Sims, Tennessee(Coldly): Are you threatening the Governor of this State with blackmail, Mrs. Carey?
Likewise the scene in Carey's office when he confronts the abusive father.
Moral Event Horizon: If Buelton's forcing Sarah into prostitution or framing of Billy for drug charges (then threatening his safety in prison in exchange for money) didn't cause the bastard to cross it, then his ordering of his posse to open fire on the people attempting to pull the tank over to Tennessee definitely pushes him over. Thankfully, the Tennessee Highway Patrol make sure the louts don't open fire.