Going to build 308, planning on18" barrel.   What's the difference between carbine length gas tube vs rifle length?  Besides the length of hand guard I could use.  I do think I'd like the longer hand guard.

You're probably better off choosing a good barrel and then going with what it has. The 18"s I know of use a longer tube. I went with the mega.

You should experience less felt recoil due to the decreased dwell time if the overall barrel length is the same.

I think I can tell what you're asking here.  I think an 18" barrel will often come with a mid-length gas system instead of rifle or carbine.  Even if available I don't think you'll want carbine...too harsh on the action, more recoil, etc.  18" barrels also come with rifle gas, so either rifle length or mid-length gas will work fine.  On the handguard, as long as you get a low profile gas block to fit under a handguard, you should be able to choose whatever length handguard you want.  Do you have any selections on barrel options at this point or just asking general questions?

Thanks for reply,  questions answered.   Looking at getting an Aero,  read good things about them.   Need to decide if a stainless steel is worth the extra few bucks or chrome lined.   Don't expect to be putting a large round count through it.