Da Vinci's Cannons

Leonardo Da Vinci worked as a military engineer before he became a painter IIRC. Now he designed some very unique weapons. But what caught my eye the most was his 3 barreled and later 8 barreled cannons, and his cannon that could adjust elevation. So its the 15th century how could these inventions become widespread? What affect could it have? An 8 barreled cannon could have a machinegun like effect, however they would be buggers to reload. What would the world be like if Da Vinci's military inventions gained prominence?
some were tried

IIRC, there were a bunch of experiments with multi-barreled guns. The overall rate of fire was less than a bunch of regular ones, as loaders would get in each other's way. Also, when one barrel was being loaded, you couldn't fire the others--the loader was in front of the whole contraption.
I haven't seen the cannon that could adjust elevation, but that could be useful. I don't recall when guns with trunions came around, but until the machineing techniques to ensure consistance with each shot are in place, the quoin (wooden wedge driven under the breach of the gun) is precise enough.
The armored vehicle is interesting, but needs a power plant.
He had some great ideas, but many of them were simply impossible with the machinery.
Had the genre existed, he could have gone int science fiction very easily--how many of today's scinece fiction gegets will be as close to reality as his were?
By 1450, neither multibarrel guns nor elevation mechanisms needed inventing. THe Burgundian artillery, then technology leader, used 'ribaudequins', small-caliber multibarrel breechloaders, for antipersonnel applications. Its medium-caliber guns - both the more common breechloading and the less usual muzzleloading designs - had ean elevation mechanism that opearted by hinging the bed of the gun to the carriage. This was impractical for larger calibers, but then, they used those only for sieges, and castles don't shift around much.

Pictures from 15th-century manuscripts also show a number of other designs that may have been used, or may just be imaginary. We know that swivelguns mounted on carts were occasionally used, but I doubt they were very common. A Swiss manuscript show a battle between boats covered with wood armour, with small-caliber guns protruding from those turtle shells. A few designs featured rotating gun platforms that always have the ready gun facing the enemy while those facing back are serviced and loaded. Leonardo tied into a common theme of his day with his weaponry designs, he just drew them better. What I find really interesting is his attention to mechanical details (but even that is not all that uncommon).

Oh, and of course he could have invented the steam engine. Anyone could have at the time. Maybe he did (we still don't know how some of his alleged automata worked). The earliest modern design is by Giovanni Branca, made in 1629 and highly impractical. What would he have wanted it for, though?
In my A Different Fate for the Templars in the Writer's forum, there's a Leonardo analogue who's hired by the Venetian Arsenale. There's also an Armenian who invents similar inventions.
In a series of novels including THE MEDICI GUNS, by Woodhouse and Ross, the emphasis is on Leonardo as a gunner. But he is more interested in researching ballistics, improving gunpowder, and devising a rangefinder than anything else.