You use the best save you can for the given situation. If your model is hit by a weapon that ignores its normal armour, then it uses an invulnerable save if it has one, or a cover save if one is available. Whatever the case, you only use one save at a time.
To be more precise, you may choose one, and only one, save to apply to any one wound. so if you get hit with a heavy flamer, a shurican catatpult, and a brightlance on a single model that is in cover and has the following saves (5+, 6++ or cover 4+) and all 3 weapons cause a single wound...
You could choose to save vs the heavy flamer with your invulerable (since flamers negat cover saves, and its ap is high enough to ignore a 5+ save), save vs the BL using the cover save, and use the cover save for the shuricat as well.
Glad we could help. Ostsol and I make a decent team when it comes to filling in all the rules on a subject and thats what the threads are all about. Good luck in your future games, and always ask away about question (after you've checked the bgb first=)
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