head space?

A: Because the trigger has been improperly adjusted.
B: Because some one botched a "trigger job".
C: Because the stock is improperly inletted, causing a trigger malfunction.
D: Foreign debris in the sear/trigger. Clean it, thoroughly.
E: The Sear and/or Trigger is severely worn, and no longer properly engaging the other.
F-Z: other posters will provide other possibilities.

Above all else: Take the action out of the stock, and clean it thoroughly. See if the malfunction occurs while out of the stock (before and after cleaning). If it does not, see if the malfunction comes back while in the stock (after cleaning).

It's not a head space issue.
 
Check the firing pin ! Remove carefully clean firing pin hole, firing pin and spring .Check for burrs etc.Lightly oil and check it on a dummy round.
 
I do not know, first I would open the bolt then close the bolt without a round in the chamber while listening for the hammer to fall as in 'CLICK'!, no click is good, then I would install a primer in a case, chamber, close the bolt then open the bolt 'CAREFULLY' and eject the case and examine the primer for any kind of a dent, mar or scratch.

Then I would remove the bolt and pull the firing pin (assembly) to the rear making sure sure the firing pin protruding from the bolt face was attached and not jammed, to cock on open the firing pin must move back when the bolt handle is raised.

F. Guffey
 
The 78 has the same adjustable trigger as the 700, my first guess is someone "lightened" the trigger beyond safe conditions and reduced sear overlap to where the firing pin isn't held back enough. Adjust the trigger back to 3lbs+ and try again (and yes, just primed cases would be a good thing to test safe function with).
 
First thing I would check would be to see if the sear is following.

As mentioned before, if someone monkeyed with that trigger mechanism, then it is possible that bolt sear is over riding the trigger sear.

Google adjusting the M700 trigger and you will find articles on how to set up one of these triggers.

I would not use this rifle until you have this figured out. It is dangerous.
 
I would strongly encourage you to bring that rifle in to a Gun Smith!
There is no place for tinkering with a trigger mechanism when not extremely familiar with that kind of work.
And your user name???
 
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