New Build, persistent jamming

[lazyengineer]

I helped a buddy put together a Grendel. I’ve done a number of Grendel and 5.56 builds and haven’t ever had an issue quite like this before. The bullets (spent or live) will jam in the chamber to the point that the charging handle alone can’t remove them without excessive force.

... Next step, we removed the barrel and put a second identical barrel on the same bcm stripped upper.

Thoughts?

My thought - it's your barrel throat. Replacing the barrel with the same barrel, means it still has the same throat.

For whatever reason, it's common for manufacturers to short throat a Grendel, so that mag length rounds with an agive that's forward of the case neck much any at all, will jam into the throat. Like - REALLY common; to the point of basically standard. 123 H's at mag-length are close on this already. Who's barrel are you using?

Try different ammo to see if they can seat and be withdrawn. Do the sharpie test (which isn't definitive). Maybe this is or isn't your problem - maybe it's just how this manufacturer cut the chamber. But with Grendel, it's so common to be short-throated, that on almost any issue be it reliability or accuracy, I pretty much auto-advise to always start looking there, at the throat depth.