Disclaimer: I am not, nor ever have been a gunner, so these are the by the book answers, and the manual quoted is from 1962. Things may have changed since then.
I read something about the drills a little while ago, and spare ammunition is carried in the event of a misfire.
Here is the text from the Canadian Army Manual of training for the M2A2 (C1) howitzer wrt blank fire and misfire of the blank. The meat of if it is 15 seconds minimum between shots on a gun and 1/2 hour pause in the even of a misfire.
I have the gun drills for 25 pdr if you specifically want that gun's drill.
Here is a RCA webpage that specifically covers the remembrance day salutes that you may find interesting:
http://canadianartillery.ca/news/gun-salutes-11nov/