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I keep seeing people have this interpretation of ruins terrain and it absolutely battles me.

I have faced players insistently claiming that when their unit is between the walls of a ruin but not touching one of the walls, they are "obscured". There is nothing about this in the rules and goes against logical sense. If you're within the walls of a building, you are clearly within the terrain and units can shoot you. Obscuring rules always talk in terms of drawing lines over or through the building, so you stand on the other side of the building if you want the obscuring bonus. They then will have things outside the ruin move up to touch the edge so that they can shoot into said ruin.

One tau player was particularly ridiculous saying this battlesuit's move-shoot-move just need to move just a tiny inch back so he's not touching the wall anymore and is now obscured.

Is this some tournament thing people keep doing? This seems ridiculous.

First of all, every ruins has a blueprint:

Core Rules page 48 wrote:FOOTPRINT
When this terrain feature is set up on the battlefield, both players must agree upon its footprint – that is, the boundary of the terrain feature at ground level. This is so that players know when a model is wholly within this terrain feature. If this terrain feature is mounted on a base, then this will define its footprint.

Secondly there is the Autumn Balance Dataslate change to the visibiliy rules:

BALANCE DATASLATE AUTUMN 2023 wrote:‘Models cannot see over or through this terrain feature (i.e. a unit outside this terrain feature cannot draw line of sight to a target on the other side of it, even if it would be possible to draw line of sight to that target through open windows, doors, etc.) Aircraft models are exceptions to this – visibility to and from such models is determined normally, even if this terrain feature is wholly in between them and the observing model. Models can see into this terrain feature normally, and models that are wholly within this terrain feature can see out of it normally. Towering models that are within this terrain feature can also see out of it normally.’

Now please review pages 11-13 of the Rules Commentary for some excellent pictures of what this means during play.

Now to answer your question, there is no Obscuring in the current rules. You either have LOS or you don't. However, you are not allowed to draw LOS out a Ruins you are not wholly within. You can't touch one side of a ruins to gain LOS past it. You have to go wholly within it. It right there on page 48 of the Core Rules, so pull it up the next time someone tries going back to 9th Edition terrain rules.

Needless to say, none of what I wrote above applies if you aren't playing 10th Edition.

That sounds a lot like common house rule I have ran into elsewhere before. It's partially to help combat with terrain that has holes while also giving ability to expose unit to shoot.

Against rules as written? Yes. Also is "1st floor blocks LOS period" which is another common house rule people adopt rather than redo every terrain piece to be solid walls(which at this point would be pretty much required to play 40k sensibly without house ruling).

But if people at your gaming area tend to play like that then that's how they play. You could try to talk about it. It's quite possible they haven't even thought that you might not be aware of it or even that it's house rule(old habit die hard).