If I join in progress horde and finish do I get credit for completion?

Knocking out the maps on master.

Progress towards the achievement? Nope, you need to be in the lobby before the game starts.

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Ok, that`s what I thought. Thanks.

Can you still start before 50 though. ?
I mean new game but starting at a higher wave ?

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According to this it looks like you can.

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It worked that way for Seriously Chapter 1 where loading wave 50 and finishing it counted towards the achievement.

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