Pcp High pressure deposit for air rifle, shear strength of materials and safety margins.

[Argie guy]

HI first allow me to introduce myself, i live in argentina, my name is Fabrizio, i don't know if is the procedure to introduce oneself but it seems to be in most forums i see. 

Anyway back to the project i wanted to make a deposit for a high preasure air to use in a pcp from tubing, the deposit needs to have a round bar of the same diameter as to have this part act as a top for the air.
The air is gonna be at 200 BAR or something like 3500 psi maximum so i need to have for safety reasons a safety margin of 5 so that materials wont fail on me.
The air is at 200 bar, the top inside the tubing is of 1.9mm in diameter, the screws wich i want to use are M8 quality 12.9 and they will work on shear force meaning they will screw on the piece inside the tubing and screw also into the tubing, "piercing" the tubing and reaching the inside piece that serves as a top, hence the reason the force applied on them is shear force, the tubing is 3mm thick on the walls.
Given all this factors, how much screws of M8 12.9 should i use to secure the peaces with a safety margin of factor 5? 

Thank you guys in advance, i am sorry i do not bring anything to the tamble as i am not an engeneer i am a lawyer and i get along pretty bad with mathematics. Thank you 

The 10.9 is the version of a little lower quality of 12.9