What you are calling the detent is actually the ejector. Ruger used two types in the 77. The early ones were the plunger type in the bolt like a Remington 700 and the later was the true controlled feed type that springs out from the side of the receiver like a Mauser or Springfield 03. If I recall, the plunger types were the ones with the tang safety. If it is the pushfeed plunger type they frequently stick. It needs to be taken out of the gun, cleaned, lubed and reassembled. To test it you should be able to hold an empty case in the bolt tight against the boltface with one fingertip on the mouth of the case. Let your finger slip from the case mouth and the ejector should flip the case away sharply. About the only thing that will make the one in the receiver fail is a broken spring. Check to see that it fully raises up into the slot in the boltface, if not the spring could be weak or dirty. One thing in common to cause both types to fail is the extractor being fitted to tight to the case. This is very rare as Ruger doesn't hand fit this part like they were in the older rifles above. Is this a new gun that did this from day one, or did it work properly and then fail???? Difficult to do a diagnosis over the web, I could fix it in seconds if I had the rifle.
