Just because they make 300 a day or not doesn't mean they are being made that ffast. It could be built(assemblyed) that fast though. I could have 10,000 rifle/handguns in a warehouse just needing a slide to frame mounted and count that as "built 10,000 guns daily".
And by "300 guns or what ever their number is really" thats not per model #, its per unit total. So I could have 20 guns in my company that people buy, and 5 of x model and 15 of y model daily made. I am sure more Shields/M&P 556 are made than 8 shot revolvers.
SHTF vs TEOTWAWKI you ask? SHTF, just some crap on the ceiling and walls, TEOTWAWKI end of the world as we know it. Orignally made by 9mm/®™©