German to English Tech/Engineering Military / Defense Military weapons (rifles)

This is my fourth (?) question about a legal dispute dealing with a rifle not shooting straight.

"Erprobungen litten unter dem Mangel, dass sie die Eigenstreuung der verwendeten Munition - ermittelt aus einem sogenannten Munitionsprüfrohr - nicht berücksichtigen".

I'm really stuck on this one. I have found German sources of this word on the Internet, e.g. one with a sentence similar to the one above but I'm still no clearer what it actually is.

any help or ideas very much appreciated !!