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Find out if your barrel is carbine, rifle, pistol length etc. they don’t come with gas tubes so you have to buy it separately. And it is a straight tube. However the question is how long your hand guard is, and from your gas block journal. My sig was a carbine length, and a 13.5 inch hand guard. I ordered the 4.3 inch one and still couldn’t easily adjust the gas system. So I ended up getting a 13 inch hand guard. Hope this helps.
Riflespeed website https://www.riflespeed.com/Choose-your-Gas-Control_ep_41.html
If you’re asking which adjustment knob length to pick, this is based on the gas system length(pistol, carbine, rifle, etc.) compared to the length of your handguard. Riflespeed has a calculator on their site that will tell you which model(s) will work for your build
Riflespeeds website has a sizing tool.