One approach that I would recommend against is replacing the hammer spring with a lighter one. I know that no one has recommended trying that yet, but it's often what people do with pistols.
When it was dirt cheap, I bought a bunch of the S&B pale blue/green boxed 7.62mm NATO ammo with the black stained primers (ZV69). In my new (at the time) SAR-48 (standard rifle), I had a fairly high failure to fire rate with this ammo with hard primers. The second strike would always fire the round.
This same ammo is 100% in an M98.
I tried replacing the hammer spring with a slightly 'heavier' one and it helped considerably, but didn't completely fix the problem.
The SAR-48 has always been 100% with the other ammo that I have fired in it, including Portuguese (which is known to be problematic this way in some rifles).
Just sayin'...
Forrest