Sorry for the barrel nut, mixed up with a TROY one.
I was saying 7" for short stroke piston because if longer (or covering gaz hole on barrel) you will have to install gas tube and low profile gaz redirector (I don't remember the name it's been years since tinkered with AR15... HK417 all the way

).
A piston needs some space to even the level for the piston rod.
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Tous les jours nous nous endormons plus instruits mais pas forcément moins bêtes...
I can do a CTRL+ATL+DEL on your gun !
Gun doc in Montreal on AEG, GBB, GBBR and Bolt rifle (gaz and spring)
Heavy custom builder (including painting)