Imagine the firepower of the Swedish musketeers multiplied by one and a half times. For Horn’s response to Tilly’s flank attack, that’s like having four and a half brigades instead of three to face the Imperial-Leaguists. Loaded with pre-packaged cartridges of cannister shot, the Swedish regimental cannon delivered devastating volleys into the enemy. Together musket and cannon chopped into those front ranks.

Three heavy cannon were assigned to the center squadron of the Swedish brigade. For the two flank squadrons there were three small three-pounders, their overall weight light enough to permit easy mobility, thus, able to be moved quickly wherever needed in a changing battle.

Although some writers have touted that each little cannon could be moved about by its two-man crew (or one horse), that pair of cannoneers had a detachment of six musketeers with them to provide the muscle to move and the supporting small-arms firepower to protect.

Once set up, those three-pounders loaded with cartridges could fire three rounds for every two a musketeer could, according to a veteran named Schildknecht from Pomerania. More than half those cartridges were cannister shot, consisting of 30-32 lead musket balls.

Devastating at close range, these little infantry-support cannons had only around 2/3's the range of even ordinary three-pounders due to the shortened barrel and the reduced charge for the cartridges.  Not to be overlooked, the ammunition supply was limited to the cartridges carried by the crew. It should be noted that if the three-pounders participated in the artillery exchange that began at noon, they were at the limit of their range (600-800 paces); in the later phase, when Horn's maneuver countered the attempted flanking move by Fürstenberg's men, the range was closer and the three pounders were in their element.

Although written years afterwards, relying on descriptions of others (since he had been wounded in his first charge), Montecuccolli mentioned this, which likely described accurately what happened after the Swedish left flank turned to face the Imperial-Leaguists:

"...[Tilly] received a horrible, uninterrupted pounding from the king's light pieces and was prevented from coming to grips with the latter's forces." - Raimondo Count Montecuccolli, Imperial officer.

pg.141: The Military Intellectual and Battle, by Thomas Barker, State Univ. New York Press, 1975.

The original intent for using cannon as infantry support was against the hard-charging Polish cavalry. As early as 1625 King Gustavus Adolphus had a 12-pounder or even a 24 pounder shooting hailshot attached to an infantry squadron. But those guns were too cumbersome to do the job well. Next came the experimental leather cannon with its limitations, garnering more attention than it deserved. The first bronze three-pounders debuted in 1629 in Prussia with barrels weighing 140-150 kilos. The pieces used at Breitenfeld had their weight pared down to an average of 123 kilos (which proved too light because they wore out too quickly – the following year, the Swedes brought in heavier, 138 kilo, three-pounders).

Incidentally, some the battles following Breitenfeld in which the Swedes lost (Alte Veste and Nördlingen) or suffered a draw (Lützen), prior to each they had left behind their little support cannons due to terrain.