Whoever has grinded the US and Japanese Tree thoroughly will easily identify that the M1928A1 Thompson (50) is not a Tier V SMG but a Tier IV one being roughly on par with the Type 2B Late (especially after the rate of fire buff)
Both guns have 50 rounds, Whilst Thompson has bit lower rate of fire, much lower bullet velocity and much higer recoil, it has superior stopping power making it better at close range but inferior at medium / long range compared to the Type 2B late. Anyone will easily conclude after playing both guns, like me that actually both are roughly on par (with their own advanatges and trade offs) and that both should be BR IV.
M1928A1 (50) has no business at BR IV
The 100 drum version is BR V material.