Russia - threadbanned users in OP

The reason the US is currently short on spare ammunition to give away is that it made the foolish mistake of hoping that other nations might have enough munitions for their own use, not because there were various consolidations and mergers in the industry. After having donated millions of rounds, the US has not dipped into its own pre-determined war stocks levels (with the arguable exception of the cluster munitions which were theoretically still war stocks, even though slated for withdrawal). The US continued to build things like Javelin missiles and M1 tanks in excess of its own requirements precisely to ensure that its production capacity did not atrophy in the same manner as European nations. Remember this, for example, from a decade ago?

How many times on Boards have we seen commentary about the size of the US defense budget and scathing commentary about the US buying missiles instead of hospitals. Who could have possibly known that you can't defend your basic existence as a nation by slinging cheap prescription drugs at the enemy?

Ammunition is a standard design, who makes it is not much of a factor. If it's all made by Lake City or Scranton, by GDLS or BAE is irrelevant. Production of the M4 rifle doesn't care if it's made by a Colt factory, an FNH factory and a Remington factory (as they are), or if they are all bought out and built by three Colt factories. It's also worth noting that the equipment to build much of the hardware is owned by the Army. GDLS may operate the Lima Tank Plant, but the plant and the machines to do it are government owned. Same with Watervliet arsenal which makes the barrels for tanks and artillery. No amount of mergers or acquisitions will change that, and even if BAE bought out GDLS or whatever, they couldn't shut down Lima no matter how much they wanted to. (They could, however, reduce the number of employees, which would lead to delays in restarting due to need to retrain).

All one needs to do is look at history. In WW2 there was a bit of an outcry in that the US government did not award major contracts to the multitude of businesses that could undertake contracts at the time. That was a deliberate decision by the industrial genius Bill Knudsen, that the government should focus only on the major corporations on the basis that the few major corporations could expand and subcontract as necessary. And that's exactly what they did. If they had to share common parts plans, they did. Continental built Wright engines, Ford and Chrysler built Rock Island tanks, and if something was beyond the ability of the company to make, they subcontracted out to companies which they eventually bought out after the war anyway. The government doesn't much care if an aircraft is made by Northrop with Westinghouse Electric ball turrets, or if the whole thing is made by Northrop Grumman Electric (Northrop bought Westinghouse's defense business in 1996). In effect, the US built its way through WW2 using a small number of providers by choice, and it worked. Today there is a small number of providers, not by choice, but there's no reason to believe it doesn't have the same effect.

The US has historically solved many of its military and manufacturing problems by throwing money at it. The US has the ammunition manufacturing capacity its politicians have been willing to pay for for the last three decades. Had the politicians decided that they wanted to buy 100,000 rounds of ammunition a year, the factories would have been set up to do it. The number of companies in the bidding have nothing to do with that.