The little parts (grips, etc.) for the gun are in abundant supply. Barrels... I know Green Mountain is the main producer, but you're telling me that nobody else produces AR-15 barrels? Getting specific models I could see as being difficult, but getting a hum-drum set-up? Come on...
Ok, you're wrong.
Just look at the idea of making more receivers. How many investment casting or hot forging molds/forms are there? They each cost thousands of dollars to make and then they require hundreds of thousands of dollars in equipment to use. How many companies want to invest in these new molds/dies and the equipment to make them?
But, let's look at he real bottleneck.The choke point for AR production is barrels, bolt carriers and barrel extensions. All the companies that make ARs get these parts from one, two or three manufacturers in the US. Production can be ramped up only so far so fast and then you have all the production equipment in use at a facility. "Buy more", you say? "Put it where", is the response. These businesses don't have a lot of excess production capacity in place and they don't have bigger facilities than needed. Having idle equipment/personnel/space is a quick way to go out of business.
"Buy more". With what? The manufacturers have to get credit to purchase the equipment and we're all aware of the current "credit crunch". Who's going to loan the money?
"Build more". With who? Where are the skilled workers to run the CNCs, heat treating furnaces, coating equipment, etc. that the manufacturers can't purchase with the current credit crunch in full swing. Where would they put them? In the buildings they don't own with the supervisors they don't have? And how do you handle all that even if you can get the credit for the equipment and facilities?
Do you just throw money at the problem and proceed with no business plan? What's the point if you turn your profits into equipment and facilities and personnel you won't need in 18 months when the demand is met? Feed the demand and then go out of business because you blew all your money meeting a short term demand?
No, it ain't "that easy".
Doublestar was in a position to pretty much move fast since their business plan was to add equipment, staff and facilities within the next 3 years as they grew. They just advanced their plan by 3 years and got to work.