It refers to using a #2 pencil eraser as a bore guage to determine for certain if the gun is chambered for .30 Luger or 9mm Luger. It seems like the eraser is specifically used, however I've never actually done it. Go and it is a 9mm, No-Go and it is a .30.
Because a new-guy might confuse a .30 for a 9.
dju
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