Anyone who has taken the time to look at military small arms has noticed that there is a trend for similarity across military’s for similar solutions during the time period.
.69 Caliber Minie Ball
Black powder arms are limited by velocity and the fact that round ball projectiles dominated personal ordinance until the introduction of the Minié Ball, limiting bullet weight per caliber, the only reliable way to increase power was to increase the diameter of the ball resulting in a heavier projectile and more striking power.
The British Brown Bess was the standard issue British musket (smooth bore, no rifling) and fired a .69 caliber ball. Of course, many of the colony militias where armed with the Brown Bess. The French supplied many Charleville Muskets also of .69 caliber to the patriots during the war. Notice the co-evolution of small arms. History is rife with hostilities between Britain and the French. Notice again both were armed with .69 caliber muskets.
.50 & Other Caliber Minie Balls
The Americans had access to rifles, that spun and stabilized a ball for accuracy out to about 300 yards versus about 40 yards for a point target for muskets or about 100 yards if firing into dense ranks of soldiers. These where on the order of about .50 caliber. Most were the long or Pennsylvania Rifle and were between .25 to .62 caliber with the majority being .45 to. .50 caliber. However, the long-range accuracy came at a price. Muskets could be loaded faster, where cheaper and rifles where more prone to fouling. The civil war was the era of the 50 caliber Minié Ball and other elongated bullets.
Musket VS Rifle
Muskets were used but rifled weapons dominated the battle field. The southern forces armed themselves with .58 caliber British Enfield Rifles whilst the industrialized north had the Springfield series of rifled muskets (muskets re-barreled with rifled barrels). The Springfield rifles where .58 caliber as well. The Spencer repeater was a .52 caliber rimfire cartridge, and the paper/linen combustible cartridge of the Sharps 1859 used by the northern sharpshooters and cavalry was .52 caliber. Except for the Henry, arguably a civilian weapon that was .44 caliber all rifles where .50-.58 caliber.
Cap and Ball Revolvers
Hand guns were similar, dominated by .44 caliber or .36 caliber cap ‘n ball revolvers. The LeMat revolver was a favorite of the southern cavalry officers. It fired 9 rounds of .42 or .36 caliber ball with a 20-gauge single shot shotgun barrel under the rifle barrel. Both sides used Colt and Remington revolvers, the S&W No.1 in .22 Short rimfire was a popular backup.
Gun Bore Sizes
Post-civil war the US adopts the .45-70 in 1873, the Brits get the Martini-Henry in .577/450 (.577 case necked to .45 caliber) and the French going to a 11mm (.42 caliber). Again, all similar. 11+mm weapons were popular during this period by many of the world’s combatants. Come the 20th century we have the 7 or 8mm Mauser, .303 British, Americas 30-06, Russia 7.62x54R and the Japanese converting to 7.7mm. All real similar in performance. Today we have the 5.56 NATO versus the Russia’s 5.45x39mm. The overall trend is smaller bores with modern centerfire rifle rounds, and in the black powder era the trend was towards the bigger bores. All dependent on the propellant.