I can't tell you about production of magazines specifically, but I have been on one end or the other of various manufacturing processes for 35 years. What we are mostly dealing with is probably injection molding like Magpul PMAGs. I have not worked around injection molding now for several years, but lets just say for simplicitys sake that a molding machine makes 8 mag halves per cycle, or four complete magazine shells. Say it takes one minute per cycle to close the mold, inject the heated material, press for whatever predetermined time, cool the material enough to retain its shape, open the mold, and eject the parts. That is 240 mags per hour, 5760 per 24 hour day if you don't have breakdowns, have to change out molds, or have three of your operators take the day off to watch NASCAR. Then they have to put in the internal parts and assemble. That sounds like a lot until you take the demand you are seeing in your area and multiply it by thousands. There are a lot of molding machines around that can be subcontracted to make parts, but the mold itself that forms the specific parts are very expensive and time consuming to produce and you don't do that for a temporary rush on the product, especially if that product has a chance of going from legal to contraban at the whim of a bunch of idiots. I suspect the metal mags are stamped out and also require a mold to form them. Then you have to get all of your suppliers to suddenly double or triple your normal demand, once again not knowing how long it will last, or if it could stop completely in the near future. Its one thing to invest whats needed if the volume is likely to continue in the foreseeable future, but quite another if its temporary. One component delayed in shipment can bring everything else to a standstil on the factory floor.
And dont get me started on being able to find temporary workers that are capable of getting themselves out of bed and dressed more than two days in a row. The good old American work ethic has become extremly rare especally at the low pay entry level, I guess they are all holding out for a "managment position". You can run your good employees into the ground with overtime and increased demands, and they too are wondering if the job will even be there in six months. You end up with FNGs running, and tearing up your equipment, more OSHA reportable accidents, Workmans Comp, and Lawsuits.
Its just not always worth it to try to keep up with a temporary increase in demand. So they just try to keep it going at full capacity until they catch up with the demand.
A small, lean manufacturing operation like Magpul can put out a lot of product but they don't have a lot of wiggle room to turn up the volume. Hope this helps explain some things.