Where can i find info on spaceborne macro cannons?
Also, given that ramming is a viable tactic in BFG, thats a sign the imperium lack range.
BFG sourcebook is your best bet, being free online at this point, given how long ago GW discontinued it.
And really, it isn't. It's more a sign of relative range, compared to speed, armor, and firepower. 40k ships are stupidly tough. As in, I think it is actually stupid, how tough they're described as. The Void Shields on the Retribution are supposed to be able to absorb the mass-energy of the Earth before failing. I always mentally tone down 40k statistics, since GW has said that all canon/lore is in-universe propaganda or writing of one sort of another. Even if it's just Craftworld Eldar scholars who decided they wanted to document what was happening and put it in the Black Library.
Still. The hugely long reload rates necessitated by having a crew reload massive shells in cannons that basically need to be inspected after every shot, which is in turn required by armor constructed of pure depleted plot-onium, and speeds high enough to close those gaps...
And while yes, xenophobia is a big factor, I feel like the bigger one that would cause difficulty is just making contact with the citizenry in that way. You have to have control of the orbit and launch war amounts to an invasion, first. Abominable Intelligence Droids are another matter that did create hatred, as would any sign of xenos crew among the Federation.
I have to say that I got a different impression about the Great Crusade and healthcare system in 40k than Kot did though.
It'll be fun to join in the firearms discussion later though.