I am surprised the 12 gauge didn't stop the fight.
If luck had been with the agents that day, it would have.
As Leadcounsel has expressed in detail, this was first and foremost a failure of planning before the encounter and a failure of tactics during the encounter. Only secondarily can it be said that it was a failure of a bullet to penetrate far enough.
I still contend that if those agents had entered the fight all armed with modern 40 cal pistols, it still might have goon badly for them. If you unholster your pistol and lay it on the passenger seat, and then crash your car and loose the gun, it doesn't matter what kind of gun it was.
But if they had gone into the fight with better tactics, AR-15s, MP-5s, and shotguns, Platt and Matix would very likely have been defeated early. These weapons were available to the agents. Since they initiated the encounter, they should have controlled the circumstances a little better.
A shotgun with buckshot has some impressive advantages, but some important limitations as well. It is a short range weapon. 30 to 40 yards, and the closer the better. It does not penetrate barriers well. But on the plus side, it is the ideal weapon to engage a rapidly moving target. For a given point of aim, a clean miss with a rifle can be a good hit with a shotgun, If/when you do get a solid hit, the tissue damage is very severe and blood loss is very rapid. Get 4 or more pellets into the lungs, and blood loss is catastrophic. Ever see a deer hit in the chest with buckshot? They bleed out like a sponge.
Unfortunately, the only time the agents were able to use their shotguns was when the two badguys were still sheltered by automotive glass, and thus it was not very effective.