If the bolt holds open on the last round, I'm questioning if it is under gassed.
Some times a tight chamber can effect ejection. Can you place a factory cartridge in the chamber with slight pressure and then tip the barrel up and the cartridge fall out without sticking in the chamber?
If you push your ejector in and out on the bolt, does it stick?
Do a gas ring test.
Do you have a bent firing pin cotter key?
Do you have excessive powder residue and carbon in the fire control group?
Do you have a rubber O ring in the extractor? Especially with the Grendel, a rubber O ring will create too much tension on the extractor..
If you remove the ejector and insert the ejector pin in it, does it swivel freely without sticking?
Remove the extractor, insert the case rim into the extractor and see if it is a tight, snug or loose fit..
Remove the extractor and ejector and put the case into the bolt face, does it insert easily or is it tight??
It could be the extractor..https://www.youtube.com/watch?v=kMrMUyyWV0k
On another note, I'm not saying yours is, but I have seen over gassed rifles behave just like your rifle..
Some times a tight chamber can effect ejection. Can you place a factory cartridge in the chamber with slight pressure and then tip the barrel up and the cartridge fall out without sticking in the chamber?
If you push your ejector in and out on the bolt, does it stick?
Do a gas ring test.
Do you have a bent firing pin cotter key?
Do you have excessive powder residue and carbon in the fire control group?
Do you have a rubber O ring in the extractor? Especially with the Grendel, a rubber O ring will create too much tension on the extractor..
If you remove the ejector and insert the ejector pin in it, does it swivel freely without sticking?
Remove the extractor, insert the case rim into the extractor and see if it is a tight, snug or loose fit..
Remove the extractor and ejector and put the case into the bolt face, does it insert easily or is it tight??
It could be the extractor..https://www.youtube.com/watch?v=kMrMUyyWV0k
On another note, I'm not saying yours is, but I have seen over gassed rifles behave just like your rifle..