*Tried searching couldn’t find info*

I’ve been sitting on this builders set and decided to tinker around in the garage. I grabbed what I thought was just a standard rifle length gas tube but after putting the upper together the gas tube bottoms out in the key before the carrier sits all the way in the upper (pic attached)

I looked at my stash of tubes and found that I had two 15 1/8” and two 15.5” length rifle tubes.

What am I missing here? Clearly I need to put the 15.125” tube in this upper but can someone explain which application requires which rifle length tube?


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