As I understand it, the "G3's" Being sold by Century are
made from C.E.T.M.E. Spare parts. The 7.62 CETME
is a slightly different animal than the 7.62 NATO.
The CETME has a longer and lighter projectile.
Thus the springs in your "G3" may be a bit too light
for a full power 7.62 NATO round!
Your bolt may be taking some punishment from recoil
with light springs. Also, the gas system may need adjusting
to let a bit less gas through (Just a guess on that one)
I have an HK91 that (Knock on wood) I have never had
a single malfunction with, So it is a sound design.
Yours may just need a little TWEAKING here & there
to get it right! There is a company offering FAL's
from spare parts also, You may find one of those
interesting also!