The firearms were found packed in individual boxes aboard a Dutch-registered truck that arrived on a ferry in the port city.
Although customs rules were not broken, the Turkish driver did not have licences for the weapons, police said.
Controls at Italian ports have been stepped up in the wake of the 13 November Paris terror attacks.
EU interior ministers meeting in Brussels last week agreed to tighten security and share more information about the gun trade to prevent trafficking of illegal weapons.
It came after attackers with automatic rifles and suicide belts killed 130 people at a number of sites in Paris. Some of the attackers had lived in Belgium.
The 781 weapons seized in Trieste on Tuesday were pump-action Winchester SXP shotguns, according to a police statement (in Italian).
Prosecutors are investigating possible charges.
The Turkish manufacturer of the weapons was cited for breaching national security regulations, Ansa news agency reported.
Belgium has historically been a European hot spot for the arms trade - legal and illegal - due to its relatively liberal domestic gun legislation.
Even though Belgian legislation is now stricter and more solid, the country has retained this reputation.

The collision involving a black Peugeot 206 happened on the B9013 Sheriffston to Orton road, near Altonside, at 21:10 on Monday.
Two men aged 30 and 19 were seriously injured and are being treated at Aberdeen Royal Infirmary.
A 23-year-old woman suffered fractures and another 19-year-old man sustained minor injuries.
The road is currently closed and police have appealed for witnesses.

The online retail giant reported a $92m profit compared to a $126m loss for the same period a year ago.
Sales also rose 20% to $23.2bn in the second quarter to the end of June, boosted by strong growth in the US.
Its shares, which fell 1.3% during the day, leapt 18.5% to $571.24 a share in after-hours trading on Wall Street.
The firm was also upbeat on its prospects for the current third quarter, forecasting sales would grow between 13% and 24% compared to last year, much higher than analysts had predicted.
Sales in North America rose 25.5% to $13.8bn in the second quarter, driven by purchases of electronic goods and general merchandise, helping to drive the strong performance.
Its cloud computing division, Amazon Web Services, also made a contribution, with sales up 81% compared to the same quarter last year.
It said its widely promoted "Prime Day" on 15th July - which offered special deals to customers who signed up to its fast delivery service - had been a success with more new members trying it globally than any other day in the firm's history.
Amazon founder and chief executive Jeff Bezos credited hard work for the results: "The teams at Amazon have been working hard for customers," he said.
Throughout its history, Amazon's revenue has climbed while its profit has hovered around or below zero.
On Thursday evening, the e-commerce company reported a profit.
As customers in North America bought electronics and others products, sales in its biggest market rose more than 25% compared to last year.
And if you thought Microsoft's cloud computing business did well, then take a look at Amazon web services which includes its cloud business - revenue soared more than 80 percent.
The only slight negative was that sales growth outside the US wasn't nearly as impressive.
Still after years of ploughing its cash into new technology , mostly aimed at making it easier and faster for customers to get what they want, the results are starting to pay off - at least in the eyes of Wall Street

The helicopter swooped on Durdham Down at about 14:40 BST on Sunday, with air crew reporting the kite was soaring 1,000 feet (300m) up in the air.
No action was taken against the man, who was told the flying limit was 240 feet (73m).
The pilot said the kite endangered small aircraft from Bristol Airport.
Italian police in the northern city of Trieste have seized nearly 800 shotguns en route from Turkey to Belgium.
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Four people travelling in the same car have been injured in a serious crash in Moray.
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Amazon shares have surged more than 18% in after-hours trading after it reported a surprise profit and better-than-expected sales figures.
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A police helicopter landed on an area of Bristol parkland to order a high-flying kite enthusiast to bring down his craft.