How many rounds did those old Tommy Gun Magazines hold?
So, I looked it up:
The Thompson submachine gun ("Tommy gun") used a drum magazine in its classic form, but the drum magazines for this weapon were abandoned on the World War II models.8 The M1921 Thompsons could accommodate either 20-round box magazines or 50-round cylindrical drum magazines; the latter were known as "L drums" because "L" is the Latin numeral for 50.9 A 100-round "C drum" magazine (the letter standing for the Roman numeral for 100) was available, but weighed more than eight pounds and pushed the total weight of the gun to almost 20 pounds (9.1 kg).9 The M1928 Navy and M1928A1 variants, used by the US Navy and US Marine Corps, could also accept drum magazines, but standard box magazines were more popular due to the drum magazines' weight and tendency to jam.10