Agreed, the seconds spent reloading allow people to flee, if they're not trapped.
I think 10 rounds is the lowest limit in any state today? NY tried for 7 and got struck down (2013?). States appear to grandgather-in existing magazines or spend years/decades fighting legal battles to actually achieve bans.
Today 3D printers are in nearly every high school. They can be had for $100. A magazine is 3 simple plastic parts with a simple spring. The CAD models for common mags are all over the internet. A middle schooler can make a 30 round AR or glock magazine in a day or less, start to finish.
Criminals will ignore but appreciate the advantage these limit laws give them. They're misdemeanors in the states I looked up. Law abiding citizens that have to and can afford to, will permanently modify or replace standard magazines (30, 20, 10 round) to comply. People that can't, won't.
The typical suburban mass shooter will simply practice changing 7, 8, 10 round magazines, and the heroic police will continue to wait outside for him to run out of ammo or targets.
I just don't think this is the place to spend limited federal political capital.