what effect do modern cannon have on a jet?  would 20mm do much damage or would you need 25-30mm?  and what is the bullet, HE or AP?

also , how does the A10 use aluimum cases, don't they fail under pressuer

Aluminum casses save some 400kg on A10 load , so makes sense to use them.

smg762 said:

what effect do modern cannon have on a jet?

devastating

smg762 said:

would 20mm do much damage or would you need 25-30mm?

As allways its all about it placement.
A 9x19 between the pilots eyes are a sure shoot down. Since modern aircraft are pack full of important and breakable stuff a single 20 mm hit can be enough.
Bad placement and not ideal ammo choice on the other hand will be of little effect.

20 mm hit on a Harrier during the Falklands war. The round either failed to explode, exploded after penetrating or it has been an AP type. The hit has been ineffective and the aircraft safely returned to the carrier.

20 mm is not really common as aircraft or anti aircraft armament. It has been replaced by more powerfull calibers and longer ranged systems. A single hit by a modern AA shell of 25 mm or lager to a modern aircraft would be an allmost certain kill.

smg762 said:

and what is the bullet, HE or AP?

The standard/legacy shell type to use against aerial targets would be a HE type with impact fuse.
There are more modern and optimised choices. Like FAP(DS), AHEAD (programmable airburst sharpnel type), HEIHC, HE-MP etc.
Not all are nessecarily available in all calibers.

smg762 said:

how does the A10 use aluimum cases, don't they fail under pressuer

As mentioned by Mr. T the intention behind the aluminum cases has been to save weight. But they do fail under pressure. The 30x173 using alu-casings is downloaded compared to the types using steel casings. They are usually not interchangeable even though the caliber and case dimensions are the same.

Unless you want to talk about the USAF's B52G that had four .50 BMGs until some time in the1990s.