The US Government OWNS the Lake City Army Ammunition Plant. Winchester currently holds the contract to run it. Historically, the contract has allowed the contractor to sell on open (citizens) market ammunition produced in excess of the government’s needs.
Other manufacturer’s picking up the slack is easier said than done. LCAAP is huge, enough to supply the entire DOD during a world war.
So if this one ammunition plant makes enough ammo to supply the entire Department of Defense, ONE MISSILE can cripple U.S. troop ammunition supply.
This falls under the category of “all your eggs in one basket.”
And if needed they can open the other ten or so government ammunition plants that are mothballed across the country but the cost would be too much. Better to take capacity from the existing 5.56 and stop selling the excess. That plant makes or can ammunition from Pistol rounds to 40mm rounds so all the capacity for production is needed to be shared among all those types of munitions.