The Biden administration's sprint to supply Ukraine with weapons central to its military success against Russia has yielded a promising acceleration of arms production, including the standard NATO artillery round, output of which is expected soon to reach double its prewar U.S. rate of 14,000 a month.

The stakes in the U.S. effort to shake up a sclerotic defense acquisition system are particularly high as Kyiv tries to claw back territory from Russian control in a slow-moving counteroffensive whose fate, U.S. officials now say, hinges on the West's ability to satisfy Ukraine's astonishing hunger for artillery ammunition.

But industry experts warn of major challenges in sustaining an elevated output of arms and equipment needed not just to aid Ukraine but to ensure the United States' own security in potential conflicts with Russia or China. Those include overcoming scarcity of key inputs including TNT and maintaining expanded capacity amid fluctuating budgets and uncertainty about future military needs.

"Whether you think it's going well or it's going poorly is whether you're a glass-half-full person or glass-half-empty," Cynthia Cook, a defense industry expert at the Center for Strategic and International Studies, said of the attempt to ramp up arms manufacturing swiftly. "But also, it's how much you work in defense acquisition."

The war in Ukraine has brought a boom for American defense firms, which are racing to expand production and factory capacity. It also has meant a bureaucratic scramble at the Pentagon to get needed equipment in time.

A year and a half after Russian President Vladimir Putin's February 2022 invasion of Ukraine, production deals are only gradually being cemented. Of the $44.5 billion the United States has appropriated for manufacturing arms destined for Ukraine or replenishing donated U.S. stocks, the Defense Department to date has finalized contracts to produce weapons costing roughly $18.2 billion, or 40.8 percent of that total.

To Cook and other industry experts, that ratio, as modest as it appears, is an achievement for the military's often slow, unwieldy acquisitions system, in which concluding a major contract often takes up to 16 months - let alone manufacturing a piece of complex equipment for use in battle.

Pentagon officials say the eventual value of Ukraine-related contracts concluded through Aug. 18 will be substantially higher than the $18.2 billion figure, largely because it doesn't account for contracts that give companies about half the expected value up front, with additional costs finalized later.

Experts say the United States, as it invests in expanding the production of munitions, drones, air-defense missiles and other arms that Ukraine needs, also must ensure that it can sustain expanded capacity as requirements evolve. After grueling counterinsurgency wars in Afghanistan and Iraq, the Pentagon has looked to fund the capacity to win, or deter, future conflicts that could require a very different set of capabilities and weapons systems - particularly against the threat posed by China's burgeoning military. But Russia's invasion of Ukraine laid bare a NATO-wide munitions crunch, highlighting important vulnerabilities in fighting the war of the present. "The question is making sure that this problem, which is illuminated now, isn't swept under the rug in future compromises," Cook said.

Defense and industry officials spoke about the race to accelerate arms production on the condition of anonymity to provide a candid assessment of the evolving effort.

The administration has focused largely on expanding output of the 155mm artillery round, which has been a mainstay of the West's conventional arsenal for decades and proved critical for Ukraine in the ongoing counteroffensive. Despite Ukrainian forces receiving U.S. training on modern combined-arms maneuvers over the winter, the government of President Volodymyr Zelensky has largely jettisoned these tactics, instead embracing an attritional, artillery-heavy approach as it seeks to breach Russian minefields and fiercely defended lines of trenches.

U.S. officials now say that the tactical shift made by Ukraine will require sustaining the country with a robust supply of artillery shells. While Ukrainian forces have created a munitions advantage on the battle's southern front by using extended-range missiles from France and Britain to strike Russian ammunition depots behind the front lines, they say those blows will prove consequential only if Ukraine also can penetrate Russian defenses.

Since February 2022, the Pentagon has concluded $2.26 billion of manufacturing contracts for the 155mm round, helping to increase U.S. output from 14,000 units a month before Russia's invasion to around 24,000 per month today. Production is slated soon to reach 28,000 a month, with the goal of producing 1 million shells a year by fall 2025. Officials declined to say what share of that would go to Ukraine vs. being held in reserve in the United States.

A host of companies have different roles in manufacturing the shells, including forging steel projectiles and assembling them for battle. The Defense Department also is investing in expanding production lines.

The pace of the munitions ramp-up could have long-lasting effects for civilians in Ukraine after the Biden administration's decision this summer to provide controversial cluster munitions, which White House officials described as a "bridge" solution until output of conventional artillery shells increases.

U.S. officials hope the cluster munitions, which consist of large pods that release hundred of bomblets - some of which fail to detonate upon impact and can pose a danger to civilians for decades - can help Ukraine maintain momentum until more conventional shells are made. In the near term, the mix of artillery ammunition being sent to Ukraine will become more heavy on cluster munitions, they said.

Officials in the U.S. Army, which is responsible for procuring the 155mm artillery rounds, are moving "as fast as humanly possible" to speed up production, Army Secretary Christine Wormuth said this month.

"We're going to be able to continue to provide the Ukrainians with munitions, I think, for a long time," she told reporters. "I think they're probably going to continue using [the cluster munitions] for a while as well."

Although Ukraine's summer counteroffensive is just months old, defense officials are already looking toward winter, when a potential lull in fighting could, as one official described, permit U.S. and allied production "to catch up and help sustain them." But Moscow will not be static, either: A break could allow Russian forces also to rearm and harden their defensive lines.

The war has been a wake-up call for Ukraine's backers across the West, where officials see an urgent need to augment their own munitions stockpiles. NATO officials have wondered how long the Western alliance could sustain a major conventional war. "No one had really asked themselves the question, well, what if 'day one, night one' becomes 'week two, week three, week four?'" British Defense Minister Ben Wallace said last month.

Wormuth, without providing details, said the United States would aim also to set its artillery reserves as a higher level. "One of the lessons learned out of the Ukrainian experience is we need to go back and revisit those minimum standards. And we may have underestimated," she told reporters this month.

Officials note that some contracts signed to replenish U.S. supplies or produce specific weapons for Ukraine have been finalized in 30 days or less, including deals to make Switchblade and Phoenix Ghost loitering drones and NASAM air defense systems. They also are employing, for the first time, multiyear contracts for munitions.

Restocking the U.S. arsenal will require finding basic weapons-making materials, experts say, a problem complicated by a global scarcity of chemicals and explosives. The United States no longer produces TNT and has since moved to a substitute called IMX, an explosive that provides power with less risk of accidental detonation.

But the dramatic increase in shell production has pushed the United States to seek out new global suppliers of TNT. Poland has been a primary U.S. source, but the Pentagon is working with its allies and partners to increase its supplies, potentially including from Japan.

The United States has healthy stockpiles of explosive fill, officials said. But as factories churn out more shells, "we know we'll need additional production of both those propellants and those explosives," another defense official said.

The war has cut the United States off from one source of TNT, as Russian forces now control an area of eastern Ukraine where an explosives company called Zarya agreed in 2020 to a multiyear deal to procure TNT for a U.S. contractor. The conflict disrupted the supply from Zarya, but officials said the company never was intended to be a major supplier to the United States.

The availability of propellant, a combustible charge that sends the artillery round through the barrel, is another constraint to sustaining increased U.S. and European production.

Martin Vencl, a spokesman for the state-owned Czech company Explosia, which makes propellant charges, noted the scarcity of related raw materials, such as nitroglycerin and nitrocellulose. The company is running at full capacity to make propellant for 155mm rounds, but long-term investment is needed to double its output, which the company hopes to achieve by 2026, Vencl said.

Camille Grand, who served as NATO's assistant secretary general for defense investment from 2016 to 2022, noted that the recent conflicts in Afghanistan and Iraq did not consume artillery at anywhere close to the rapid pace as the war in Ukraine has, meaning that suppliers were not forced to tap so deeply into their stocks.

"We're all relearning what it means to do mass production of ammunition, which had become a . . . nonstarter" for many NATO members, Grand said.

Grand attributed some European countries' paltry ammunition stockpiles to the preference for funneling limited defense funds to big-ticket items such as jets and main battle tanks.

"No defense minister would put on a T-shirt saying 'I bought stockpiles and spare parts,'" Grand said. "They all want to be the guy who said, 'I bought the last fighter aircraft.'"

European nations are trying to remedy that problem. This summer, the European Union approved a three-track plan ultimately to produce 650,000 rounds of large-caliber ammunition a year and committed itself to delivering 1 million rounds of artillery ammunition for Ukraine in a joint effort within the next 12 months.

Grand said the biggest obstacle is the timeline. "It's good and nice to know that five years from now, we'll be able to ramp up production and refill stockpiles," he said. "But in the meantime, Ukraine is running short, and we're going to be in trouble."

Experts say it is important to avoid what one defense official called a "boom and bust" cycle by ensuring that Western militaries continue effectively to signal a demand for these weapons. Failure to do so could result in factory lines going cold, as occurred with Stinger missiles, with the manufacturer of the shoulder-fired missiles having to enlist retirees to help get production going again.

The challenge goes beyond accelerating near-term production. The Pentagon needs to "continue to procure at that level over a longer period of time so that we have not just healthy stocks, but a healthy production and industrial base that's able to meet them," the second defense official said.

"We want to make sure that we're able to maintain focus across the government, and really across allies and partners on the need of maintaining just consistent high demand for these weapons," the official said.

A senior industry official familiar with the Pentagon acquisition process, who spoke on the condition of anonymity to offer a candid assessment, said the bureaucracy continues to struggle with articulating evolving needs, sometimes leaving defense companies to make hiring and investment decisions with incomplete information.

"The Defense Department does not have a very good track record of communicating requirements," the official said. While it's clear that producing artillery rounds is a priority within the agency right now, the official said, "the question becomes how strong the commitment is over fiscal years, over presidential administrations, and the administrations of other countries."

Continued high levels of U.S. funding for Ukraine, which has enjoyed generally strong bipartisan support, may face increased opposition as a small but vocal minority of Republican legislators questions the wisdom of the commitment to the current fight. The Biden administration last week requested an additional $20 billion in security, economic and humanitarian aid for Ukraine.

So far, Republican leaders have managed to defeat attempts to curtail aid by critics within the party, including Rep. Matt Gaetz, R-Fla. "The Biden administration is sleepwalking our great country into a world war," he said on the House floor last month.