What gas system do you run in your 3G AR?

Well since my 2/3 gun guns are 7.5" 5.56 and 8" .300blk, the only one that fits

Top choice amongst serious competitors is rifle length DGI on an 18" or 20" barrel. I have two match rifles configured this way.

For matches where a shorter rifle is an advantage (hoser matches) I have both 16" and 14.5" barrels with midlength DGI.

Anything shorter than midlength is less preferable due to harsh recoil. I have 7.5" and 12.5" SBRs that I run for fun (12.5" with suppressor for night matches), but they are definitely niche guns with severe limitations in a typical 3-Gun match situation.

Piston systems just add weight with no performance benefit for 3-Gun.

Rifle length piston system

Currently running a carbine gas as I'm adapting a home defense gun to 3G and will switch out to middy  on a 16 at some point but I have a hard time noticing the difference from a carbine vs Middy.