One thing I'd like to point out, is that numerous tanks throughout WW2 carried turret-mounted versions of the same guns that the gun-teams were fielding. Granted this was mostly found among the Soviet and German (and other Eastern Front Nationality) tanks. But still the fact remains that the Tiger mounted the same 88 that field-gun crews were firing, with exceptions to how it was mounted. Other such examples of this include the T-34/85's 85mm main gun (the Soviet 85mm being an Anti-Aircraft cannon, originally), as well as the SU-76m mounting the ZIS-3 76mm field gun. And that's saying nothing about the 'Self-Propelled-Artillery' vehicles like the ISU-122 and ISU-152, which were little more than an IS-2 chassis with a hull-mounted artillery field gun.
In those regards, I personally think the Auotcannon Predator turret is the most sensible turret-design of 40k Marine vehicles. So often you see twin-linked gun turrets that look ridiculous from a design standpoint, simply because it would leave no room for the crew or gun housing within the painfully small turret. And seeing how the Autocannon is not a man-portable weapon at all (note that it is a gun-team for IG, not unlike WW2 field guns), it also makes sense in that regard as well. Marines are just different in that they are supposedly big enough to carry their own miniaturized versions of the Imperial field-guns into battle, even if it means apparently still being unable to fire it while at a run.
On a side note, almost all 40k tanks are infantry-support. Few (if any) are actually designed per the rules to engage other tanks to an adequate degree of success. The Lemun Russ Vanquisher is about the only one in that is a moderate AT weapon (Annihilator being a close second), but is still horribly outmatched by other tank-destroyers like Multi-Melta Land-Speeders, Melta-Hounds, Melta-vets in a Vendetta, Combi-melta Sternguard, etc.