I noticed you mentioned 3 gun as a possible use. Almost all the 3 gunners I know run 18" barrels with rifle length handguards (and rifle length adjustable gas systems) and use DI rifles. Most will take the extra mile to have adjustable gas blocks tuned to the lightest load, use lightened bolt carriers and buffers, ect... to minimize recoil. just about everything opposite to "mil spec" in terms of the supposed rules for AR reliability, hehehe. With that said, most of their rifles run like tops.
If you're looking for a do it all type rifle, I'd say a 14.5" or 16" barrel, medium gas system, with rifle length handguards would get you what you want. Run a good BCG and a H2 buffer and that setup should run just about anything your throw in it. It may not be the most finely tuned rifle like what the 3 gunners will use, but it will be rugged, reliable and do anything you'd pretty much want to do with it. With that said, if you really want a unicorn rifle like the Jack Carbine or a KAC, LWRC, ect, then go for it. Just be sure it's really what you want
If you're looking for a do it all type rifle, I'd say a 14.5" or 16" barrel, medium gas system, with rifle length handguards would get you what you want. Run a good BCG and a H2 buffer and that setup should run just about anything your throw in it. It may not be the most finely tuned rifle like what the 3 gunners will use, but it will be rugged, reliable and do anything you'd pretty much want to do with it. With that said, if you really want a unicorn rifle like the Jack Carbine or a KAC, LWRC, ect, then go for it. Just be sure it's really what you want