I didn't have the internet or unlimited money, I had to make do with available codices and what I could find from surfing the internet from my school's computer lab EIGHTEEN YEARS AGO, and lo and behold, none of the sources I could reach had anything like an actual caliber for the autogun.
Edit: Let alone a series of documents about an obscure experimental German munition from W.W. II, as the munition we are discussing is a caseless form, not a full bullet and shell.
Further edit: I'm not trying to be combative here either, I stated that the crunch and fluff have changed in the intervening time, and it is definitely possible that I missed something when researching that would have changed the equations. But I've got twenty pages of math on every single factor I could find as a point of comparison, and that showed that overall 40k's weaponry would be right about equivalent to modern weapons of similar class, so if you have better data then by all means I'll accept refutation of my numbers.
I'm not a ballistics specialist, I don't have a degree in physics, and I've only studied laser technology passively, so I will fully admit that my knowledge is limited.
Even further edit: Okay, there is straight up no such thing as an 8.25 mm caseless munition outside of the autogun, you cannot use that as a point of comparison to anything real world, we have no concept what it's ballistics, powder load, configuration, or any other point of reference look like for determining anything useful beyond it being a solid-shell ballistic weapon of roughly equivalent power to other large-caliber battlerifles. And designing the munition from the ground up as a thought experiment amounts to baseless supposition and is therefore not even remotely useful.