I've noticed that the default ammo is incendiary ammo. Or at least it is always checked when I go to the loadout screen. Any idea on the effects?

Is it less accurate?
Shorter range?
More of a chance to cause flamers? I imagine this one is a no-brainer. But I am not sure of the above.

I have noticed it seems to cause more damage to ground targets.

I suspect the "more damage to ground targets" is more your good shooting on a stationary target then choice of ammo.
OBD can jump in here but in my multiple conversations with them there is no difference in damage or anything else between the two ammo choices. Not range, not ballistics, nothing.

The "incendiary" setting just enables tracers. When I asked a "OBD insider" I was told it was to simulate early war/no tracer and late war/always tracer. That started a conversation about when tracers started being used in MG ammo.
IIRC nothing was set in stone and I looked it up but now forget! Being a 1917/18 guy I use tracers always.

Anyway perhaps the language of the two settings can be changed in UE.

Oh, OK. Makes sense.

I've seen more damage when on a ground attack mission lately. I used to empty a belt and see nothing and now I see damage. Perhaps it is the better gpu and higher settings.

I can't believe it's better marksmanship. Shooting at planes the stats are around 15%. That has never changed.