BRUSSELS (Reuters) - NATO is set to issue its first ever defence industrial pledge at its Washington summit on Wednesday, pushing countries to boost arms production and return to a stricter standardization of ammunition to make shells interoperable on the battlefield.

"Ukraine has shown that our standardization was good on paper but not so good in the field," said a NATO official, speaking on condition of anonymity and referring to munitions such as 155mm artillery rounds which are in short supply globally amid soaring demand triggered by Russia's full-scale invasion of Ukraine in 2022.

While NATO enforces standards for rounds fired by small arms such as assault rifles, enabling allies to use each others' munitions, it is not so easy for artillery shells.

Although there is a NATO standard for artillery ammunition, its implementation is voluntary and a lack of adherence has fragmented the market and hampered the flow of supplies.

Fourteen NATO nations have reserved the right to deviate from the standard, meaning there are different types of 155 millimetre ammunition.

The different rounds can still be used in all howitzers but operators need to enter the specification of the shells when loading them into the weapon or risk them missing their targets by as much as 50 metres or 60 metres (160 to 196 ft), according to artillery experts.

The specifications are registered in tables that operators use but officials say companies sometimes do not provide all the necessary data - something NATO aims to change. It also aims for more common standards, which would mean simpler and shorter tables.

"A world in which there was one standardized NATO round, where every ally produced the same thing, would be a much simpler world for military commanders," the NATO official said.

"Because then you have absolutely no doubt everyone's using the same round. So you could take munitions from this dump over there and move it to these howitzers, and that would be the most efficient way of doing things."

Still, NATO may run into opposition from munitions makers as such a move could raise competition and lower prices.

With a view to boosting arms production, especially for "battle-decisive ammunition" such as artillery shells and air defence missiles, leaders will agree to report to NATO annually how they intend to meet the alliance's defence targets and how they aim to increase arms production, the official said.

Western countries have been scrambling to backfill their own inventories, often neglected since the end of the Cold War, while also supplying Ukraine with desperately needed weapons.

NATO allies have stepped up the production of ammunition since 2022, from roughly a few 100,000 artillery shells before the war to some two million rounds this year and some three million rounds expected in 2025, according to the official.

(Reporting by Sabine Siebold; Editing by Josie Kao)

By Sabine Siebold