It doesn't. 300 blackout requires a new barrel. It's 'trick' is that all of the other components don't need to change.

PLEASE look here regarding safety issues:

https://news.ycombinator.com/item?id=32389075

it doesnt fit the bore of the barrel, it 'fits' the chamber.

if you are using .300 blackout and .223/5.56 nato be very carefull or do something like colour your rounds with a sharpie

[edit] by the way there are very few dumb questions, asking a question is far more intelligent than acting like you know it and blundering through it by luck.

for your average hobbyist who doesn't actually have to worry about camouflage of anything, do something like wrap bright orange hockey grip tape around any magazine body that contains .300 blackout. And some totally other color on 556.

exactly that sort of thinking.

there is a color coding system for ammunition:

https://www.bulletpicker.com/pdf/MIL-STD-709C.pdf [PDF]

indicating special function, such as armor piercing; incindiary; fragmentary HE etc.

it is also a popular option to have receivers colorized or stamped. alot of magazines are polymer and could easily be colorized, and detailed, for instance with a large pictogram of the intended ammunition.

this is also on a little more of an angle, a reason to not let that combination of, persons you dont trust; .300 blk; and 5.56 chambers to occur.

not exactly milspec, but I have also seen custom laser engraved dust covers on AR platform stuff that has ".300 BLK" or "5.56 NATO" etched into both sides of the cover, in addition to whatever markings might be placed on the lower/upper receiver bodies.

It requires a different barrel. As mentioned, lower receiver, bolt carrier, magazines can be reused.