What gas block are you using Gos, I assume a fixed gas block as it sounds like a complete upper from JSE. An 18 6.5 Grendel with this type of failure can be over gassed. Many times the symptoms of over gassed and under gassed are the same.
If you have a 5.56/223 PMag you can usually get three to five rounds to reliably feed to maybe eliminate the possible mag failure issue. What type of buffer tube, buffer and buffer spring are you using? If your not using an adjustable gas block and you eliminate the cause as being a mag Id probably buy an adjustable gas block.
If your using a carbine length buffer tube, carbine spring and 3 ounce buffer. Have you tried a heavier H-2 buffer or put the upper on a lower with a rifle length buffer tube, rifle buffer and rifle buffer spring?
If you have a 5.56/223 PMag you can usually get three to five rounds to reliably feed to maybe eliminate the possible mag failure issue. What type of buffer tube, buffer and buffer spring are you using? If your not using an adjustable gas block and you eliminate the cause as being a mag Id probably buy an adjustable gas block.
If your using a carbine length buffer tube, carbine spring and 3 ounce buffer. Have you tried a heavier H-2 buffer or put the upper on a lower with a rifle length buffer tube, rifle buffer and rifle buffer spring?