Space marine equivalents in terms of training from the Imperial Guard do not have rank-and-file BS 3. Not only does this support the argument, but it underlines that BS is the result of intense training, which both space marines and guard elites have, and not superior genetics.
Now, here's something I would find far more fluffy and plausible, for those who want their space marines to be crack shots:
Put a combat tactics and targetting computer into power armours and interface it with the marine over their black carapace, which in this case should be the equivalent of an MIU. The result is, while helmet is on and their systems active, the marine can use the superior vision enhancement etc. of his computer interface to locate targets easier, the servosystems in his armour to correct minor problems while aiming etc.
Why do CSMs not attach integrated weapons systems as auxiliary and direct them over MIUs, if they want to be truly shooty? They wear, what the heretek has to integrate and usually with a power supply that can support it...and no Omnissiah telling them it's heresy.
This has always baffled me in every FFG 40k RPG: Why does the high tech power armour not truly make use of its potential?
It's not all genetics. Superior equipment is a huge, huge space marine factor. Bolter damage is just the most nonsensical way to go about it, tbh, when you have a high tech toy you're wearing on your very own body.
Short version: Marines should have full MIU benefits for their power armour, if those systems are functional, and be able to attach integrated weaponry to it.
An example can be found here: http://youtu.be/7MLoXQIPPNI?t=57s
Edited by DeathByGrotz, 15 August 2014 - 12:48 PM.