I don't think you have much of a chance taking on even a minimally trained group by yourself.
I think you have two choices:
1) Bunker up under defensible cover and hope that after you cap the first one that approaches your position, the bad guys move onto easier prey.
2) A fighting retreat to get the hell out of there.
But, you're fighting someone equally competent as you are.
An aside: Something's been irking me about most gun forum folks, recently. Seems the average gun forum frequenter at TFL and such has a budget is about $600 for a gun, $20 for a cheap-*expletive deleted*ss "gun bucket" holster that fits (poorly), and $100 for ammo for the new gun. Then it's time to start saving another $600 so he can buy the next thing on the front cover of Guns & Ammo next month.
There isn't:
-$500 for ammo
-$100-$200 for range fees for the next 6 months to a year
-$250 for a couple training classes
-$100 for a GOOD holster or two
-$150 for more magazines and a mag carrier
-another $500 for ammo
I think most of us on this forum are past the gun-wooby phase (or never got into it in the first place) and know how to use our firearms with proficiency that meets or exceeds most law enforcement, military or competitive shooter requirements to be considered "proficient."
Consider an enemy, or a dozen to twenty enemies, at that same level. Who have trained to that proficiency. It isn't that hard to get there. They aren't obsessing over the "grip angle" of one gun over another or the MOA difference between an AK and an AR. They just get whatever works, spend $1000 on ammo and get a fanatic with extensive combat experience to train the cell out in the woods or desert. That know they can pick up secondhand body armor at gun shows for pennies on the dollar, and some creative tailoring can make a vest that won't break the bank and will probably work for several hits. They know how to use primitive $25 microcontrollers, household chemicals and 9v batteries to make 20oz soda bottles into bombs, and deploy them in the garbage cans around the mall or other theoretical target. That know they can repeatedly recon the facility, strolling down service hallways and learning the building INTIMATELY with little fear of actual reprisal, since no one stands up to trespassers or random folks wandering around where they shouldn't be. They know they can park vehicles in a way that block the alternate service or fire escapes of a building.
They REALLY plan this out.
You can't just "pick off one or two of them" because they aren't any more likely to be picked off than you are.
If you hole up in a store and are known to be a threat, they'll gladly throw mustard or ricin gas into the store to flush you out, or just use a molotov to set the clothing store on fire. Or, they'll use flashbangs and an assault. I know I can find flashbangs to buy. I'm sure other folks can, too. They don't want to live through it, they just want to kill as many Americans as possible. Your "success" in fighting them off would be an affront to their dignity and would represent a loss to them. They would pour more resources into your situation than most others, knowing that your melted pistol clutched in your charred skeleton would be a symbol of conquest to their followers.
If YOU were to assault a large public facility like a mall, you would expect at least half a dozen concealed weapons holders and/or cops to be there, right?
So,
1) gets you burned.
2) gets you cornered if the service exits are blocked, or herded to the killing zones if you head to the main exits.
Sit down and REALLY THINK how you might take 10-20 guys and massacre a public attraction.
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Now... how do you survive it?
It's like playing chess against yourself, but giving your "opponent you" a two move head start.