When DHS issue the contracts they do not take it all at once. They sit back and wait . They then place a order for a large number of rounds, that disrupts the supply. And cause short term shortages of components.
Then they sit back and wait for a chance to throw another disruption in the market it works well. The reason they effect other calibers is components. There is only so much brass, copper power on the shelf at any given time.
The production shifts to meet that demand.
When The contract for M4 was extend to remington and the order was for 24,000. Remington did not start making every part needed they went out and bought up inventory of Mil-spec parts they would need to meet the order.
While they were not buying every AR made they caused a major shortage of inventory on barrels, stripped lowers, lower kits ect. Just as the the supply was recovering private sales went nuts. The two emptied out just about every part in the system.
Do not kid yourself DHS has a big effect on the market and it is their plan to do so. Just like you do not need to own 51% to control a company more like 13% you do not need to buy it all to effect ammo supplies.