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I'm thinking of making a feral ork army. I was wondering if it would be tournament legal to use some fantasy ork models and sub them for 40k stats.
Example: Feral Orkz use boar boyz. Could I get a bunch of fantasy boar boyz models and simply give them regular ork warbike stats?
It seems like a whole bunch of the fantasy ork models would work for a 40k feral ork army list. I mean, that's pretty much what feral orkz are anyway, right? Orkz that havn't discovered tech yet?
Victory is achieved through mettle.
Glory is acheived through metal.
Overall that should be good, as long as you clear it with your opponents before the game starts. Though, in questions about most tourneys (especially local ones), to get the best answer you're going to have to ask your tourney organisers. We can only give you an idea, they'll know for certain.
When it comes to friendly games, go for it. Undoubtedly, some tournaments will deny you. Sub'n fantasy for space orks is fairly common and the vast majority of players won't care. I did the same thing, converting my fantasy army over.
Lots of the units substitute well, such as a katapult for a lobba or a spearchukka for a kannon. I'm working on converting my boar chariots into rokkit buggies.
For WYSIWYG'ing, there are some problems. Orks have slugga pistols, and it doesn't make much sense that they're shooting without them. Boars lack dakkaguns that bikes have (one of my frustrations with this codex is that you cannot remove the option of these guns, making boars less wysiwyg). With some simple conversions, they'd probably even be tournament legal.
One thing I did was adding slugga pistols onto alot of my fantasy boyz, either being held or strapped to the hip or back of the models. Some still don't have sluggas, but they're mixed into a majority of slugga-wielders.
Also, round bases are helpful. They actually can be an improvement for fantasy orcs while making them more 40k friendly at the sametime! Just put them on circular bases and make a movement tray out of cardboard with circular holes cut in it for the boy bases to slide down into. Its actually easier to rank them this way because they can rotate around instead of being constrained by the squares!