Can you push the cartridge all the way into the chamber by hand? If so, it could be a problem with the bolt binding. Since that is a slightly tapered, un-necked belted-magnum cartridge that headspaces on the belt (in a cutout at the rear of the chamber) the only problems I can think of that might cause your problem would be:
- An improperly cut chamber (doubtful.)
- Ejector binding (early M77's used a plunger-type ejector.)
- Something interfering with the fit of the case belt into the cutout.
- A bolt that is binding in the receiver for some reason.
- Ammo that significantly exceeds the SAAMI spec of 3.340" COAL.
Eliminate the bolt itself by trying to push a cartridge in by hand.
(Disclaimer: I am not a gunsmith.)
Also, check this for a potential safety issue: http://www.ruger.com/pdf/m77.pdf