http://www.darkreign40k.com/weapons-and-armour/rules-for-shield.html
so here are the unofficial rules for shields. but the one thing that i find illogical is that they all have 'primitive' trait, even 'modern' (in context of 40k) shields that arbites use in law keeping. at first glance it just doesnt make sense that those items would have their APs halved against wh40k weapons. why would they?
the main problem is that these custom rules see shields from two perspectives:
1) they are normally used as shields as intended by the dh rulebook. in other words, they arent used for 'soaking' damage, but for deflection in the form of parrying. so they are used actively. they have no passive use.
2) some of them that are large enough and designed with that purpose can be used as cover. so, they can be used passively. you just take them as an object, put them in front of you and hope they will be thick enough. just like armour.
the main problem that i encountered in previous arguments is that shields arent neccessarily weapons as the game would make you think by default. as i said, i think it is entirely possible to use shields with modern design and that they lose primitive trait in regards to the cover. since the primitive trait doesnt apply to regular cover, why would it be applied to shields acting as cover?
Essentially that's what people are doing with shields.