1) Platt was a hard motherfucker, a proficient rifleman with a deathwish.
2) His Mini-14 wasn't even full-auto, the dude just had an excellent trigger finger.
3) The biggest lesson to learn is that no pistol would have improved the situation for the Feds, if you bring handguns to face an expert rifleman with nothing to lose, you're disadvantaging yourself.
There was this strange idea that maybe a 10mm Pistol, a later .40S&W, or .45ACP pistol, would have improved their chances of surviving, but that's not at all the case.
Nothing short of .44 Magnum or .45 WinMag would have seriously improved odds, and that's only in theory; handguns like that are difficult to shoot and require substantial amounts of practice and getting used to, power means little if you can't place it where you need to.
They literally had access to MP5s, M16s, and better body armor, but the best they brought was ONE pump shotgun, which was taken out of the fight early, and wouldn't resume until the end, where Platt (and Matix) had already bled a bunch from all their pistol shot wounds.
Adrenaline and determination can carry you far before you eventually die, especially on top of proficiency, and pistols are generally just mediocre wounders, even in slightly larger calibers. Rifles consistently offer more power, more shots, easier control and aiming, they're just the best for fighting.