Lets see... 6.5 centimeters = 2.559 inches.

Would this do?

'Original British RML 2.5 inch Jointed Mountain Cannon - The Screw Gun' can be yours for just $32,995.00 USD, from IMA:

IMA

Item Description
Original Item: Very Few Available. Antique mountain cannon, fully functional, not de-activated. Finally approved by the U.S. BATF as an unrestricted antique; this is an exceptional, rare and very desirable British Empire "Screw Gun" dated 1902. The Screw Gun was a British development from the 1870’s. An artillery piece was fitted with a two-part barrel that "screwed" together for use. The idea was to make such a substantial weapon transportable by mule train in mountainous regions.

It took a minimum of four mules to carry just the gun and mount. Two mules each to take half of the rifled barrel, one mule to carry the mount and a forth mule to carry the pair of 36" spoke wheels. Additional pack animals were required to transport the ammunition, which consisted of bagged powder and separate studded projectiles with quill-fused primers. There has always been tremendous romance associated with these cannons of the Empire. The English storyteller and poet Rudyard Kipling even penned a lengthy poem dedicated to the Screw Gun. Transport by Freight Collect from IMA Warehouse (our mule caravan).

Every example in our inventory has been expertly cleaned, restored and painted by Curtis Wolf of Ordnance Research who has been involved in the firearms manufacturing field for more than 30 years. His work in restoring antique ordnance is world renowned as being second
to none.
The Ordnance RML 2.5 inch mountain gun is a British rifled muzzle-loading mountain gun of the late 19th century designed to be broken down into four loads for carrying by man or mule. It was intended as a more powerful successor to the RML 7 pounder Mountain Gun. Some writers incorrectly refer to the 2.5 inch gun as a "7 pounder" because it also fired a shell of approximately 7 pounds, but its official nomenclature was 2.5 inch RML.

In 1877 Colonel Frederick Le Mesurier of the Royal Artillery proposed a gun in 2 parts which would be screwed together. The Elswick Ordnance Company made 12 Mk I guns based on his design and they were trialled in Afghanistan in 1879. Trials were successful and Mk II with some internal differences made by the Royal Gun Factory entered service.

The gun was a rifled muzzle-loader. Gun and carriage were designed to be broken down into their basic parts so they could be transported by pack animals (4 mules) or men. The barrel and breech were carried separately, and screwed together for action, hence the name "screw gun".

The gun was used in the Second Boer War (18991902) on its standard mountain gun carriage, and also with the Natal Field Battery at Elandslaagte and Diamond Fields Artillery at Kimberley on field carriages which had larger wheels and gave greater mobility.

A major defect in the war was that the gun's cartridges still used gunpowder as a propellant, despite the fact that smokeless cordite had been introduced in 1892. The gunpowder generated a white cloud on firing, and as the gun could only be aimed using direct line of sight, this made the gunners easy targets for Boer marksmen as the gun lacked a shield.

It proved to be ineffectual and outclassed by Boer ordnance and was replaced by the BL 10 pounder Mountain Gun from 1901.

Either 4 or 6 guns (sources appear imprecise) were returned to service from Southern African garrisons in 1916 and were employed by the Nyasaland-Rhodesian Field Force in the campaign in German East Africa. Writers who refer to "7 pounders" in WWI are in fact referring to this 2.5-inch (64 mm) gun.