Why does my microsoft word want to change

"Consider an ideal case of a cannon firing a projectile..."

to

"Consider an ideal case of cannon firing a projectile..."

What am I missing? The first one is singular object and singular subject, the next one seems to be mixing up number. Why is the first one wrong?

Well, they are both right (kinda). "Cannon" is both the plural and the singular, though Cannons can also be used. I think word is trying to be smart here and failing. For the second sentence to be correct it would need to be "...case of cannon firing projectiles"

I'm guessing it has something to with "cannon fire" being it's own phrase...can't explain it technically but something like "they fell asleep to the sound of cannon fire" being similar to "they fell asleep to the sound of cannons firing"...I don't know which one is right/correct though.

Word does odd things sometimes that are grammatically incorrect but make some kind of sense to the program analyzing your writing.

Word does odd things sometimes that are grammatically incorrect but make some kind of sense to the program analyzing your writing.

Thanks, I was hoping something like that was the case. I have never used a grammar check function, didn't realize it sometimes made errors.