>>64180267
>The primary difference between the two is the spitzer bullet
That helps, but you're not going to catch up to 7.92x33mm by loading a spitzer in .30 U.S Carbine, when you neck that cartridge down to a 40gr .22 caliber FMJ spitzer to create .22 Spitfire (aka 5.7mm Johnson), that DOES significantly improve its performance, but it's still 30% less powerful than 5.56x45mm NATO, owing to the lighter bullet and lighter powder charge.

.30 U.S Carbine is a small rifle cartridge made for defensive use at close ranges, and closer analogues in my opinion are the FN P90 and H&K MP7, which are not assault rifles.
7.92x33mm Kurz, 7.62x39mm, 5.56x45mm, and 5.45x39mm, these are all medium sized infantry rifle cartridges intended for full on infantry combat, intended to reach out as far as grunts realistically will fight.