Good question. Your friend is broadly correct in the sense that the term "musket" during the 16th century referred to a weapon designed specifically to defeat armor with a very large bore and a very long barrel. The musket first introduced by the Spanish was typically over 5 feet long, fired a 2 ounce lead ball, and could weigh over 20 pounds, requiring that a forked rest be used to support it's weight while firing. However most troops at this time continued to carry much smaller and lighter matchlocks known as arquebuses or calivers which were cheaper, took half as long to reload, and light enough to fire without any forked rest or from horseback. The general trend during the 17th century was for armies to stop dividing troops into "arquebusiers", "calivermen", or "musketeers" and instead arm all their troops with a uniform "musket" somewhere between the size of the 16th c. heavy musket and the 16th c. caliver.

That said, below I'll discuss how the power of these weapons actually compared.

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The Origin of the Musket

At the start of the 1500s "arquebuses" could come in a wide variety of sizes, some with bores of up to 20mm or more, but most were made fairly small to keep their weight and risk of bursting down. More powerful small arms existed but generally in the form of wall guns which used a large hook or a swivel mounted on a castle wall or wagon to help absorb their great recoil.

The Musket most likely originated among Spanish soldiers sometime around 1521 described as essentially a relatively small swivel gun which was turned into an infantry weapon by giving the soldier a long, forked stick to support its weight. At the battle of Pavia in 1525, Paolo Giovio mentioned claimed that the Spanish infantry were using a new kind of firearm which was heavier than the arquebus and could sometimes kill two french men at arms with a single bullet.[1] In 1536 the musket was included into the spanish tercio system with orders that each company of arquebusiers have 10% of their men armed with muskets to help improve firepower. The heavy musket was greatly preferred by most military thinkers during the second half of the century due to its far greater range and power (Writing in 1592, English mercenary Humphrey Barwick claimed that the musket was effective against men in quality armor at 200 yards, common armor at 400 yards, and unarmored men at 600 yards [2]). Over time the ratio of muskets to arquebuses/calivers theoretically increased to 1/3, 1/2, or higher. In practice though heavy muskets remained very troublesome for most soldiers. fighting with them for long periods was extremely tiring due to their weight and severe recoil, and were especially cumbersome in a skirmish when a soldier had to dart from cover to cover while shooting. They were very tiring to march with for long distances unless the infantry was supplied with extra wagons for their guns, and in some cases soldiers on campaign reportedly grew too weak from sickness and hunger to even lift their muskets in the first place.[3] Additionally, they remained very expensive, a decent military musket in 1599 could cost as much as a full pikeman's corslet, complete with helmet, pauldrons, and vambraces, or about 3-4 times as much as a caliver.[4]

Caliber

check out the tables on this wikipedia page if you want a reference for converting pounds of lead to inches, mm, or grams: https://en.wikipedia.org/wiki/Gauge_(firearms)

The "Spanish" musket of the 16th century was typically built to shoot a bullet weighing 2 ounces, or 8 bullets per pound of lead.[1] This kind seems to have been held in the most esteem by Elizabethan writers, but other muskets could be built to shoot any where from 8-12 bullets per pound of lead. [5]

Pistols and arquebuses continued to be bored to shot 20-30 bullets to the pound or so. [5]

The word "Caliver" appeared in the mid-to-late 1500s as the term for a relatively large arquebus with a relatively long barrel.[3] Their actual bore size varied but was likely around 20 bullets to the pound initially with the whole weapon weighing around 10 lbs. In English sources, "caliver" generally becomes the standard term for the longer arquebus used by infantry, while "arquebus" becomes the term for the much shorter arquebus used by cavalry who shoot from horseback.

Occasionally there were "half muskets" in use as well, which are weapons somewhere between the size of a full musket and a caliver, but light enough to be used without a rest. [6]

In the second decade of the 1600s Maurice of Nassau created a new standardized musket design which was 4.5 feet long, weighed 16 lbs and was fired from a rest.[6] It was bored to fit a lead ball weighing 10 to the lb but fired a standardized bullet weighing 12 to the lb.[6][7][8] Using bullets which were a couple sizes smaller than the gun's bore was already the norm for most militaries during the 16th century, but when calibers are mentioned it often isn't clear if they are referring to the size of the bore or the size of the bullet. Some 16th century soldiers may have carried two types of bullet molds with them, one which fit tight in their gun's bore and one which was a couple of sizes smaller for quicker reloading.[9]

The dutch standard was soon adopted by many other European nations as well and a 12-bore bullet continued to be the standard for muskets throughout most of the 1600s.[6][7][8] According to Englishman James Turner, "In the year 1657, the King of Denmark agreed with five Colonels of us to furnish our Regiments with Muskets of a bore to recieve a ball, whereof fourteen should go to one pound of Lead. I confess this bore was too small for a Musket."[8]

The Swedish army continued to use a 10 bore musket with a much shorter barrel well into the 18th century.[6] By 1700 the British had largely settled on a 11-bore barrel and a 14-bore bullet for their "brown bess" and the french had settled on a 14-bore barrel and an 18-bore bullet for their flintlock fusils. Aside from their slightly larger bore, most of these weapons had become similar in size and weight to the late 16th century arquebus/caliver.[1]

Powder

Comparing the powder charge of muskets from different periods is tricky. 16th century muskets typically used far more powder relative to the weight of the bullet than later ones and had longer barrels, but over time the quality of black powder improved as well. Small arms in the 16th century were typically fired with a powder charge the same weight as the bullet.[3] By the second half of the 17th century the recommended charge had been reduced to 2/3 the weight of the ball for "common" powder and 1/2 the weight of the ball for "good powder".[8] In the early 19th century that had gone down to only 1/4th or 1/5 the weight of the bullet.[10]

We have no records to show what the muzzle velocities of 16th-17th century firearms with period charges of period powder actually would have been. The earliest data we have comes from the inventor of the ballistic pendulum, Benjamin Robins, in the mid 18th century. Robins tested a .75 caliber musket loaded with "about half" the bullet's weight in powder and measured that the bullet was moving at 1700 fps, about 1.5 times the speed of sound, shortly after leaving the barrel (beyond that, the speed dropped quickly over distance due to wind resistance).[11] As the quality of gunpowder available increased and the quantity per shot decreased during the late 18th-19th century, ballistic pendulum experiments generally show that smoothbore muskets loaded with a military charge continued to reach 1500 or 1700 fps in muzzle velocity.[10][11][12] Until the minie bullet was introduced, anyways.

This trend might imply that earlier matchlock muskets with their larger charges aimed to have a muzzle velocity of at least 1500-1700 fps as well, but we don't know for sure.

These tests were all carried out in ideal conditions however. In the field if a musketeer found himself with slightly damp powder, low quality powder, or powder not tailored to the barrel of his weapon it may have been much less powerful. [1][10][13]

Furthermore poor burning powder tended to cause more fouling and overheat the barrel faster. An 1842 springfield musket tested at washington arsenal in the mid 19th century had no problem being fired 100 times in a row without cleaning.[12] By contrast, many in the 16th century recommended that an arquebusier or musketeer fire no more than 10 shots per hour to keep his weapon clean and cool.[3] William Garrard writing in 1591 claimed if a soldiers firearm started to overheat during a lengthy firefight he would need to start using far less powder per shot to prevent the barrel from bursting.[14]

Supply shortages might also force soldiers to use smaller quantities of powder, which the Swedish army had to do during the 1709 invasion of russia. According to Christopher Duffy, as a result of this the Swedish volleys at Poltava "sounded muffled like gloves clapping together."[15]

It might be that 16th century soldiers sometimes thought to charge extra powder if up against a particularly well-armored opponent. Roger Williams in 1590 mentioned that overcharging cavalryman's pistol might allow it to pierce better but warned that this might cause the shooters hand to shake so much that hitting anything becomes impossible.[5] In 1846 however, N Bosworth still noted that the accuracy of an infantryman's musket could be doubled by using less than half the typical military charge of the day.[13]