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So I know the general consesus is that 30k is NOT allowed in 40k.
But, I'm looking for where this is written. I only own the first two HH books and I dont' see it in there.
There is a FW Q&A that really only disallows LOW and Primarchs (see below).
We have a league starting soon and there are several players that want to bring Mechanicum and Legion lists.
Other than an obvious Organizer ruling, I'd like to know/find if this is written somewhere. Book and page ref would be greatly appreciated.
Perhaps the Mechanicum book, which I don't have, has some special line resticting it to 30k.

Here is the latest Forgeworld 2013 FAQ:
Q: Are the armies and units in the Horus Heresy books by Forge World meant to be used in
games against regular Codex armies, such as say Grey Knights or Orks?

A: While Forge World’s on-going range of Horus Heresy books and their game content are all
designed to use and be compatible with the Warhammer 40,000 rules, they have been fine-tuned and
focused on playing battles in the milieu of the Horus Heresy rather than in conjunction with the
Codexes representing warfare in the 41st Millennium, and this will remain the case.
Designer’s Note: This means that while you are, of course, free to have fun and play games against
your friends using any forces you like, and Horus Heresy forces will be broadly ‘a fair fight’ with
Codex forces of the same scale, certain rules anomalies and inconsistencies may be thrown up that
you have to deal with, although these should not seriously affect the game in most cases. (For
example, certain units, such as those with the Stubborn special rule are at a premium costing in
Horus Heresy armies over their regular Codex counterparts, owing to the results of play testing
within their own sphere.)

In terms of using Lords of War and the Primarchs, however, these are definitely not intended to be
used in standard Warhammer 40,000 games, but only in games where both sides use the Age of
Darkness Force Organisation chart, and the specific provisions within, and in games of 2,000 points
or greater.

Designer’s Note: So if, for example, you wanted to play a battle representing a narrative where the
Sons of Horus Legion fought Orks or Eldar during the Great Crusade, you could quite easily use
those xenos forces’ Warhammer 40,000 Codexes (possibly house-ruled to accommodate larger
squads) to proxy for their Heresy-era counterparts. In this case, however, both sides should be using
the Age of Darkness Force Organisation chart, with the army’s own Apocalypse level units and flyers
available as Lords of War entries following the guidelines found on page 184 of Betrayal.

If you and your opponent want to play a cross over game then sure play it. The whole fun of 30k is that it is an other game, and that it keeps out most of the new crazy 40k cheese.


Automatically Appended Next Post:
You don't need a written rule that you can't use models grom game X in game Y. This would be like looking for a rule that you can't use space marines in a wfb game.

"30k" is an Expansion for Warhammer 40k. If you're not using the Expansion, you shouldn't be using the elements of it (including the different army lists). If you ARE using the expansion, Forgeworld's guidance, per your own quote, is that you should feel free to include non-Heresy Era lists, but that they should be using the Age of Darkness Force Org Chart along with all the other tweaks that make 30k slightly different from 40k.

In other words, you're not going to find anything saying 30k isn't allowed in non-30k games. That's not how a permissive rule set works. We're told what we CAN do and not what we CAN'T. What you CAN do is use 30k armies versus 40k armies using the 30k rules.

It's not ruled anywhere that it's not allowed. In fact by their own words it's the opposite this is an excerpt from the description of the back in print "red army book".

"The Horus Heresy Legiones Astartes: Crusade Army List is a 96 page hard back containing the complete profiles, special rules and wargear, compiled and updated, for the core Legiones Astartes Legion Crusade Army List for Warhammer 40,000 and the Battles in the Age of Darkness gaming expansion."

It specifically distinguishes both game types, so to me, that would be clear evidence of "Okay" to use rules wise, while fluff and Narrative does not support it.

I have seen the 30k version of d weapons and I like it a lot better than 40ks.

How is the FoC significantly changed?

+1 HQ and +1 Elite

I play 30k against 40k opponents on a regular basis. Unless you start to play at 2.5k and above, the 30k armies won't disrupt or change your playing style very much...if at all. About the OP, it's an expansion like any other in 40k, including Stronghold Assault. So I say play with it...adds some change and streamlined-ness to the game.