Your silencer seems to be made of 1-sided surfaces (?), so making them 2-sided will fix the problem.
But I would suggest adding a little bit of thickness to your mesh :
Select your front surfaces :
Perfrom a "divide loop" (it's a plugin from Dennis, search for it if you don't have it yet) :
Now select all the vertices of that new "ring" (select 1 vertice then another next to it and press "L") and scale up by 10% till you have something like that :
Select your surfaces again :
