In some cases this might be true, and the Astartes' weapons might be bigger, might have bigger clips, be heavier, cumbersome to use, more exotic, have more upgrades and Storm- and Combi-weapons might be commonplace due to the lack of regard to weapon sizes.
That's how I always pictured it, too. Their weapons themselves might be armoured so as to better withstand the stress of close combat, magazines holding a couple more rounds, not to mention additional subsystems built into the frame.
I noticed that a lot of people are kind of "dumbing down" the Space Marines to "stronger and tougher", their armour to "it's thicker, duh" and their guns to "they do more hurt" - yet they completely dismiss the many small advantages built into their bodies and their wargear, from implants whose only use is highly circumstantial to biomonitors, recyclers and drug-feeders in the armour to biogenetic locks and autorepair systems in their bolters.
The Space Marines are the Emperor's scalpel, and although some Chapters have become bastardised by ongoing geneseed decline and the loss of knowledge, their equipment and their very bodies reflect this. If you just want something tougher and deadlier, get a tank regiment from the Imperial Guard.
At the DH level, 1d10+5 pen 4 is capable of one- or two-shotting human enemies
If you're fighting the 10 Wounds unarmed Imperial Civilians from the core rulebook maybe ...
The average damage of a DH bolter is ~9.5, so anything that also has critical levels (like the players do) won't be as impressed .
But of course this may be my as of now rather well-developed dislike for Toughness Bonus and an ever-growing preference for the more lethal/realistic damage model in Inquisitor , so I'm certainly not unbiased anymore.
I'm pretty sure Black Industries/FFG has made caliber distinctions at least in the earlier books
But the mechanical comparison is on the same level as DH and DW having different stats for genestealer enemies. They're separate game lines not designed to be compatible (unlike FFG's new separate Star Wars lines, which are supposed to be completely compatible). What's challenging for DH is a cakewalk for DW; what's hard for DW is too lethal for DH. Even if they're described as being the same caliber I don't think there's a good way (or good reason) to unify stat blocks between games that are not intended by the developers to mix.
To address the OP, there is frankly no good way to integrate; DH characters will always be skill monkeys that obviate skill-related challenges that might trouble Astartes, and Astartes will obviate combat, if not due to the rules giving them super-stats, then with the RAW for Squad Modes and Cohesion. Either that or certain DH characters in Ascension will obviate the need for any party beyond themselves.
I absolutely agree on the games being separate lines, though I do oppose the notion that it would be impossible to "reconcile" them. Another company - Games Workshop itself - already succeeded in doing so, and the only reason this isn't working here is because someone decided that Space Marines need to be even more powerful - albeit I'm willing to consider this was perceived as a necessary fix for the effects of Toughness Bonus. One more reason to do away with it, I say.
It comes down to a unified system having to be created with the understanding that radical changes may be necessary to reign in the effects of certain core aspects of the rules which simply cease to work elegantly when pushed beyond a certain threshold.
An example of an easy fix would be these changes to starting characteristics and advancement. A somewhat more (but not too) complicated modification would be a different injury mechanic like so . There have been dozens of ideas like that tossed around here, though. Some people love 'em, others disagree on it being a problem in the first place. Matter of preferences - but for a unified system, I feel it would help to lower the gap between the characters.
I was under the impression that BI/FFG's stuff counts as fluff on the same level or lack thereof as GW's, so the different weapon levels are in fact now part of fluff.
That's how I see it (and how it was explained by several people who worked on or with GW), too. Of course, the only thing this means is that we now have multiple versions of the fluff to pick from - and some might prefer the original material to the new ideas of an outsourced studio, especially where (as mentioned above) they even contradict themselves, kind of forcing you to come up with your own solution.
My Deathwatch group still plays with GW's fluff (-> the Deathwatch being owned by the Ordo Xenos rather than being a completely independent entity), too ... as an unintentional benefit, we have thus maintained consistency with the new Codex Inquisition. It comes down to whose universe you'd like to feel part of, with what kind of books you came into the franchise, and plain old taste.
Edited by Lynata