Is midlength 18"?  That is the only thing  i can think it might be.   Carbine = 16, Midlength = 18, rifle = 20"?  Is that correct?

Different gas tube lengths

so what is better?

Quoted: so what is better?

That debate rages on!    Generally, you'll hear that mid-length is "better", smoother cycling, better longevity of the BCG, etc.  And while I agree technically that that's true, ALOT of us run carbine-length gas systems with no problems.

If you haven't seen it yet, there is a search feature on this forum.  The mid versus carbine thing has been done pretty recently at length yet again, so you should be able to find it.  

Gas systems include pistol, carbine, midlength, intermediate, and rifle.  Generally that is:  <11.5, 14.5, 16, 18, and 20.  However, there are a number of exceptions, such as mid 14.5 and 18, and carbine 16.  The general rule is that if a bayonet fits correctly on it, it is the preferred length.  There is really no true rule, though.

Search for more info.

height=8

Quoted: If you haven't seen it yet, there is a search feature on this forum.  The mid versus carbine thing has been done pretty recently at length yet again, so you should be able to find it.   Gas systems include pistol, carbine, midlength, intermediate, and rifle.  Generally that is:  <11.5, 14.5, 16, 18, and 20.  However, there are a number of exceptions, such as mid 14.5 and 18, and carbine 16.  The general rule is that if a bayonet fits correctly on it, it is the preferred length.  There is really no true rule, though. Search for more info.

I did search but did not come up with anything, sorry.  can a midlength be a 16" barrel?

nevermind

Quoted: I did search but did not come up with anything, sorry.  can a midlength be a 16" barrel?

Yes.

ETA:
www.ar15.com/forums/topic.html?b=3&f=4&t=385378
www.ar15.com/forums/topic.html?b=3&f=12&t=384703
www.ar15.com/forums/topic.html?b=3&f=118&t=382773
www.ar15.com/forums/topic.html?b=3&f=4&t=378985
www.ar15.com/forums/topic.html?b=3&f=118&t=378904