Did anyone watch the bolt while you were trying to shoot it? Sounds like a short stroke - the bolt doesn't get far enough back to completely extract the shell, so the spring pushes the bolt back forward.
As diagnosed above, most likely either gas tube alignment issue or the gas hole in the barrel needs to be bigger, giving the bolt a harder shove. Also as mentioned, more common on shorter barrels/gas systems.
The 50 is on my relatively short list, as I can see them getting banned sooner than later. Lots of other stuff on my gotta-have-it list as well.
We'll have to get together and sling some lead in the future.