ejection port size

[Variable]

FWIW,IIRC, and all that stuff: Bill A. Was the one who stated it originally on the old site. He mentioned that not every AR upper was exactly to spec, and that some had tighter ejection ports. He said that a regular AR upper receiver would work fine as long as it was to the larger side of spec, or some such.

Personally: I've never encountered a receiver that had too small of an ejection port for the Grendel, but I haven't tried every single one. FWIW: I can't recall anyone ever posting about having any issue along those lines either.