The short answer is - they weren't. Assuming that by 'historical cannons' you mean the common European and Chinese cannons built and used from 13th century to late 18th century, then their manufacturing process did not involve bore drilling or involved it to the extent lesser than it is used nowadays. It should be also remembered that modern entertainment programs dealing with historical reconstruction should always be taken with a grain of salt - they are usually designed to produce result in matters of hours and minutes (what is normal for us due to common presence of advanced technology) but original processes that are recreated could have originally taken days or even weeks while still being perfectly acceptable to a medieval craftsman or his later counterpart. Additionally, I believe that Mythbusters in the quoted episode used a small hand-held battery drill, a sub-par tool for the task.

Historically, two different processes were used to manufacture European cannons. The first one, quite quickly abandoned but enjoying a popularity in 15th century, especially in England and, to some extent, also in France, was based on forge-welding longitudinal iron bars into the cylinder that became the barrel that was later reinforced with wrought rings running along its circumference. Such weapon was theoretically easier to manufacture, as it did not require a complex form and furnace assembly. Iron was also much cheaper than copper required to make bronze guns. Precision was of secondary concern, as commonly used stone projectiles were not entirely spherical and thus lacking tight fit by design. The downside of such weapons, some of which had size that is still impressive today (with Mons Meg cannon in Edinburgh being a prime example of technology) was its proneness to a sudden, catastrophic damage, extremely dangerous to the crew (bronze guns usually signal problems by a small surface fractures or initial deformation (distention) of the barrel, wrought iron band cannons ruptured suddenly.

The other process that was used concurrently with iron forging was bronze casting that was still in use until 19th century. The process was quite complex and not dissimilar to a process of bell-casting, quite commonly known in the middle ages. Initially, the wooden model was made out of wood and then was tightly bound with rope and clay to create a barrel shape. In case of larger cannons a shaped piece of wood was used, not unlike a gigantic lathe shape bit. When the barrel was done, pivots, transport eyes, decorations and other smaller elements were added (depending on complexity, they were made of clay or wax). Ready model was dried and covered with separating agent, commonly a mixture of pitch and coal dust (water and egg white was also used) and encased in clay. When the clay dried thoroughly, it was reinforced with iron rings and bars, while the model was carefully removed from the form.

This was quite probably the most crucial and complex process because failure at that point could compromise the integrity of the cast cannon. Destruction of the model also meant that each cannon had to be made from scratch individually what added to the cost. When the form was ready, a model of the barrel bore was made - Vannoccio Biringuccio in his work 'De la pirotechnia', published in 1540 describes the process consisting of making a clay model turned on a large lathe using the metal rod as an axis. When finished and dried, the model of a bore was put inside he model of a gun and only then the molten bronze was cast inside. When the cannon was removed from the form, the bore was already present and only had to be reamed to remove any imperfections. Given that bronze is a relatively soft alloy, this was nowhere as hard as full drilling and boring used since late 18th century. The fact that the artillery was generally smooth-bore until 19th century only helped.

The technology have undergone significant changes in 18th century. An expensive and time-consuming complex, one-use forms were displaced by multi-part forms that were used for the first time in France in 1760's, concurrently with Wilkinson's sand-casting technology in England. We have accounts from 1770's stating that the French Marine Arsenal foundries on the Indret isle used multi-part sand-casting forms with models made of bronze or copper that could be used multiple times. Additionally, 1770's saw also transition from casting of cannon with a bore (a process described in the previous paragraph) to the casting of solid barrels to be drilled. This process was introduced in England in 1774 (guns from this period are usually known as Armstrong-Frederick pattern). Advent of steam power and large-scale factories only made the process faster and more efficient in 19th century.