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
cite please
Already cited by 'Wexit-Delecto' in this post's comment thread.
"Cannon salutes to persons. (1) The time interval between rounds in a cannon salute will be three seconds except as follows: (a) When honors are rendered at funerals, in which case the cannon salute will be fired at five second intervals. (b) When such interval is in conflict with prescribed safety regulations for the type of cannon employed. (c) When such interval is not feasible considering condition of ammunition and materiel, number of guns available, proficiency of gun crews, atmospheric conditions, and other local limitations."
Edit: & as 'Jlin' stated - they have two people with stopwatches for a reason...