I know that I am a little late to the party, but here are some different lengths of gas tubes.
https://truenortharms.com/ar15_default_store_view/gas-tubes-for-ar-15-ar-10-stainless-1.html
rifle length 15.675"
https://andersonmanufacturing.com/am-10-gas-tube-rifle-length.html
rifle length 15.5"
https://rockfiresportsinc.com/product/ar-15-ar-10-rifle-length-gas-tube-15-25-stainless-steel/
rifle length 15.25" but also says its for an AR-15 also so I'm not sure that it would properly clear the barrel nut