He didn't have 1300 rounds of ammo.
https://usnews.nbcnew...nds-police?lite
Listen to what she says in the news conference:
"If all of the magazines that we collected were in fact loaded fully, something on the order of about 1,300 rounds, and that's an estimation".
She is doing math based on the number of mags they collected, NOT the actual tally of cartridges. The news media ran with the ball after COMPLETELY misunderstanding what she said, and -- weird -- the police never bothered to correct the errant reports.
Anyone who has had a 500-round case of .223 ammo delivered to their house knows it is just not in any way realistic to carry around 1,300 rounds, regardless of if they're in magazines or not.