Hello Rising Sun,
I’m trying to avoid ridiculous hair-splitting here, which this issue does not merit in any way; 7.62mm seems close enough to 7.7mm for someone to understand the fact that Japanese airplanes mounted this size–which we often called .30cal as well–and that this appears to be what has pierced the 40mm. The weight figure is deliberately approximate and based upon John Campbell, which is all I had at hand & cared to grab. For those whose minds require more minute details, I will leave that digging up to them, although in any case it has no bearing on the authenticity of this old image.