spez. This may be fake news. It seems like the interval between shots doesn't really matter and it's the total number of shots that matters. Also the total number of artillery guns doesn't matter either as long as 21 total shots are made. Presidents get 21 shots while others will get lesser number. For example, VPs or foreign dignitaries get 19 shots.

sauce. https://en.wikipedia.org/wiki/21-gun_salute#United_States

https://twitter.com/reaHJFarnsworth/status/1353482161962115072

https://www.youtube.com/watch?v=xiNAly9CKSs - Biden, seems like 3 guns and 10 second intervals is correct.

https://www.youtube.com/watch?v=eFX_7IR91Vk - Trump, 4 guns and 3 second interval