If only 1 in 10 fails to fire, odds are that the firing pin in the bolt is ok. Odds are that the hammer spring is ok. Odds are the magazine spring is ok (its feeding ammo after all). And, odds are that the bolt is not closing fully. The only way you'd get zero indentation in the primer is from a bolt not quite closed. There is a safety feature which prevents the firing pin from moving unless the bolt is fully locked in.
#1 - field strip and clean the action. Pay particular attention to the recesses in the receiver where the bolt locks in. Make sure the chamber is clean. Make sure the bolt is clean. Depending on the ammo, you may have old powder residue and carbon fouling in the locking area which needs to be removed. An M16/M4/AR type chamber brush won't really work quite so well since it will be reversed in the chamber (the rod feeds in through the muzzle on an Mini-14). That's ok, just get the big bristles into the locking recess part and give 'em a good spin around a few times, then a few more. Thereafter you can use a 45cal brush in the chamber, or a 41cal brush if you can find one.
That should do it. There is also an odd chance that the ammo has the bullet seated out too far, and that his keeping the case from fully moving forward into the chamber, and thus keeping the bolt not quite locked into position. Switch ammo if that is the case.
Mini-14's hardly ever give any grief, and back in the day.... I once fired 500 rounds as fast as I could change magazines, in an attempt to see if I could get it hot enough to cook-off the round in the chamber. I was unsuccessful. And as far as I could tell, that tough Ruger barrel was none the worse for the wear either. Never was a tack driver, maybe 2 inches at best at 100yds, but no worse after the torture test.
