How much is the skill penalty for each bullet (in burst) after first one?
No skill penalty.
Each bullet has the same % to hit as 1st.
So if you have 95%, each bullet will roll 1d100,
If one bullet will roll 1-5 it'll miss and every next bullet will miss as chain breaks.
There're different chains for different targets if your bursts hits more than 1 target.
Also critical chance is for whole burst, not per bullet, if 1st bullet rolls critical, then every other bullet will do critical damage, if not, then whole burst not critical.