As it was for one of my former editors, who was fond of letting anyone who misused that [or the terms pom-poms or shrapnel] that the only time the plural of cannon should conclude with the letter *s* was when it was in a quotation, or in conjunction with the plural term *artillerymens*, as per: The artillerymens pulled their cannons over the hill.
He seemed quite pleased with me when I picked up the John Beidler journalism award for 1991, for which the accompanying presentation plaque includes a small brass cannon. And if I ever get another one of them, I'll have two of those little cannon.
-archy-/-