The only place 300 round magazines are found is Hollywood. (Good thing Alec Baldwin didn’t get his hands on one)
I have a 300 round magazine for my flintlock, unfortunately though, I’m only able to load and fire one round at a time.
They do sell 100-rd drum mags for the AR. A fully-loaded 300 rd mag would add like 6 lbs. to the weight of the rifle ... no thanks.