I remember blowing up a P-51D with just 21 rounds of the pea-shooters and without cannons on the Bf-109F4. Of course, this was done at a distance of 68 yards
.303s would do enough damage just as much as any 20mms,
IF, you are a superb flyer and can get real close enough... but considering all those 350mph+ on deck late war monsters around in MA... You won't get much chance getting within 400 yards unless you totally surprise someone. At least, since the Spit's relatively slow compared to many in MA.
So, Zyg mentioned it good there
. Try setting .50s rather than .303s if your shooting preferences are with snap shots/deflection shots... giving as much damage in a short time if it hits. Take the lead shots with .50s and don't use cannons unless you feel that it is a lead shot you can't possibly miss.
The cannons come in handy when you outmaneuver the guy with a Spit and you are lying right behind their 6 at 300~500 yards. 240 rounds of cannon ammo
isn't as pathetic as the 150 rounds, or 60 rounds of cannon on 109s, but compared to the thousands of .50s on US craft, it ain't much either. Save it and use it well