Your firearm needs attention and is not safe to go to the range before ensuring that its safety system works.
If you have detailed stripped the bolt, cleaned the bolt firing pin channel and other pieces thoroughly, and checked each piece of the bolt for chips, gouges, broken keys on the firing pin end, etc. then assuming that you have the military safety, you may need to fit a new safety/firing pin/cocking piece/sear. It can be done by yourself if you know what you are doing, this information is tucked away on the internet including pictures, or simply get Jerry Kuhnhausen's shop manual for Mausers as a guide (recommended anyway if you have Mausers).
The Mauser safety system is designed to physically block the firing pin from contacting a cartridge by positively levering the firing pin out of position where it cannot physically touch the cartridge primer even if dropped (if you have detailed stripped, that is what those notches etc. around the firing pin collar are for--burrs/damage on these surfaces can prevent movement of the firing pin to safe condition). The safety lever itself has gentle beveled curves to allow for using it as a lever against the cocking piece to pull the firing pin back from a forward firing position when on full safe. Burrs on the safety camming surface or a worn camming surface can prevent the safety lever from doing its job in pulling back the firing pin. A worn or tinkered-with cocking piece (the cocking piece-sear engagement has a specific design for safety ) as well as the sear surface on the trigger assembly can allow the firing pin to be too forward to pull it back with the safety lever. Botched trigger jobs can often be the culprit of this--people remove too much material from cocking pieces, safety surfaces, and/or sears to make the trigger smoother which results in an unsafe rifle. The point is that the sear, cocking piece, safety lever (eg safety), firing pin, bolt channel, etc. work as a unit to keep the rifle from firing in the safe position despite being dropped, etc.
If what I say appears to be gibberish--then take the firearm to a gunsmith that can work on old rifles as it is not drop safe in its current condition and might even fire when closing the bolt. The middle position is not really for firing safety but rather to unlock the bolt and allow it to be removed from the receiver.
