Mig 9 I-300 vs FS and engine question

[The Stalker]

Hello,

I hope someone who knows jets in IL can explain a bit these two things:

1) In the game, what is the difference between I-300 and FS model? Wiki says I-300 were just a couple of prototypes and FS was serial production. Are there any differences except the skin?

2) Can someone please explain what happens in the engine when gun exhaust gets sucked in? As to why does this exactly damage or flame out the engine?

Thanks!