Current British Military documents define SAA as "all types of ammunition below 20mm used with small arms".
I can confirm that, having been browsing through the Ammo Hall at Shrivenham this week. I noticed a info poster defining SAA as exactly that. So that's what is officially taught at the UK Defence Academy. That doesn't amount to Holy Writ or a legal definition of course - practice in other countries may vary, and definitions can certainly alter over time.
The same applies to the distinction between "cannon" and "machine guns", as I discussed in
Rapid Fire:
"The current usage of these terms has only become generally accepted since the Second World War. Before then, the term "machine gun" was used to describe a relatively small-calibre weapon normally firing solid projectiles while larger weapons were called "automatic guns". The name "cannon" was in English usage an obsolete term for artillery, which by that time were known as guns or howitzers depending on their function. The situation changed in British practice with the selection for the RAF of the French Hispano
moteur-canon in the late 1930s. Anglicised as "cannon", the name became adopted for the Hispano and subsequently for all other automatic shell-firing guns of 20mm or more calibre.
Different nations had different practices; in Germany, automatic weapons of up to and including 20mm were known as machine guns (
Maschinengewehr) with larger calibres being known as cannon (
Maschinenkanonen), leading to the MG and MK prefixes. In addition, designations based on function were used for particular applications, whether or not the weapons were automatic. A gun intended for the anti-aircraft role was called
Fliegerabwehrkanone (sometimes given as
Flugzeugabwehrkanone or
Flugabwehrkanone), for the anti-tank role
Panzerabwehrkanone and for mounting in armoured fighting vehicles
Kampfwagenkanone; FlaK, PaK and KwK for short. The BK (
Bordkanone) designation was used for very large calibre (37+mm) airborne cannon intended for ground attack.
Nomenclature also varied between services. In Sweden, the Air Force called their m/39 12.7mm and 13.2mm guns
automatkanon, presumably because they were larger than usual for aircraft guns at the time, while their Navy called the m/32 25mm a
kulspruta, (which translates as machine gun), presumably because this was a small weapon by naval standards.
Since the war the definition of a cannon as any fast-firing automatic weapon of 20mm or more has become generally accepted, at least in NATO."
I should now amend that a bit: US practice is to use the term "cannon" for the barrel of any large ordnance, including tank guns, artillery and mortars.