3 Rifle shot salute or 21 Gun Salute for Memorial Day services?

[Ol Gabe]

Sirs, Staff & Etc.,
This Office has read many fine reviews about the varied Memorial Day Services held across this Grand Nation of ours on Monday last. Many Organizations used a Color Guard firing 3 Rifle shots in a paced sequence followed by Taps which has always seemed to be 'the norm', some reports told of Odd-numbered Uniformed Ranks firing 21 Rifle shots followed by a Bugler while larger Services were more fortunate to have an Artillery Staff, Gun Crews, Etc., and have a full 21 Gun Salute complete with a Band, Bagpipers, Etc., as part of the Memorial Service and Honorarium to the Departed.
In searching today for an 'Official' standard regarding the use of the 21 Gun Salute at a Service of this type, the US Navy website indicated, as did others, that the 21 Gun Salute is presented by Cannon/Gun as a 'Welcome' Salute to Heads of State, President, Etc., and also ONLY at Noon on Memorial Day as an Honorarium when the Flag is at half mast. No indication/rule as to using Rifles to replace Cannons/Guns has been found, is there such a 'rule' or has this just become part of the cultural sea changes we have been experiencing in the recent past wherein old, established 'standards' have been changed to fit the moment due to any number of reasons?
Obviously there are not enough Cannons/Guns and Crews to be everywhere, so perhaps that is the simple reason and answer in a nutshell, but this Office thought it worth a query and hopes that the question is taken in the inquisitive manner it is posited and that all comments will be of an Educational and Positive refrain.
I remain, Etc.,
'Ol Gabe,
Bvt. Capt.,
Adjt.,
Dept. of the Missouri,
GAF
c/o Dunbar Station, Iowa Territory