I know this prolly belongs in the AR15 discussion, but they might beat me up in there!  I searched and couldn't really find any defining parameters for a "middy" rifle.
What makes a midlength a midlength?
How long does the barrel have to be, or is it available in different lengths?  (like the M4 can be had in 14.5 and 16" versions)
The gas tube is the same length as a rifle right?
Why are there "midlength" and "rifle" handguards if the length of the gas system is the same?  How long are they respectively?  Can they be interchanged?
What buffer and spring would you use in a midlength?  I would assume using a collapsible 6 position stock would limit you to a carbine type buffer and spring.
Do you call a midlength a carbine or a rifle?

I was planning to build a second M4, only after putting off plans to build a sniper type rifle with A2 buttstock.  But then I realize I can sort of have both in a midlength and keep as many evil features as possible, including having a usable bayonet lug.
My knowledge on all the various AR variations is quite limited, I think I have a handle on A2 and M4, but if you can steer me in a good direction I'd appreciate it!
 Thanks in advance

Midlength is normally used to describe gas port location.  Barrel can be any length.

You have carbine (short), mid (medium), and rifle (long).  I don't have the exact measurements.

Each has a specific gas tube and will need specific handguards (excluding some FF tubes).

I couldn't tell the difference from shooting my 16" carbine  barreled rifle and the 16" mid that I had.  I think it's mostly personal preference.  I'd go with a mid if attaching a bayonet is a factor.

CHRIS

FWIW- In my excruciatingly limited AR15 experience (I'm working on that!), I dont like the short sight radius of the typical carbines.  When I get my ebr, it will be a middy or better.

From what I can tell, and I'm sure I will be quickly corrected if I'm wrong, a carbine gas port is approximately 7 incles from the barrel nut.  A rifle (traditional 20") gas port is approximately 12" from the barrel nut.  A mid-length gas system is about 9" from the barrel nut or "mid-way" between the carbine and rifle system.  Hence the moniker "mid-length".

You could, theoretically, have a mid length gas system on any length barrel though I theorize that  if you have this system on a barrel shorther than, say, 14.5"  you may have function issues as the muzzle would be much closer to the gas port, reducing dwell time and making it necessary to open the gas port a bit to make it function reliably.

Hope this helps (hope I'm correct too)

(also hopes this is accurate):  apparently there are issues, whether actual or perceived, with reliability and wear and tear associated with carbine length gas systems.

its probably more ammo sensitive in using suppressed weapons or special loads for reliability.
just my guess.

also, there were some discussions on added wear due to higher pressures in the carbine system with the short gas tube.  i don't know from personal experience - just what i have gleaned over in the AR15 forum.

calling Red w/ xringprecision:  if he doesn't chime in, IM him and ask him.  
then post it for us to see .  Red is as good a source you will find for such info.  

I appreciate the effort.  Having put more research in to the matter I did find that despite what I was told by a dealer on the EE that the midlength does have its own roughly 9" gas tube.  He kept saying it was rifle length and that confused me.  Also confusing the matter is the bushmaster dissapator.  Its rifle length handguards on a carbine length gas tube.  They don't offer a midlength that I know of.  
Carbine is roughly 7" handguard and gas tube
Middy is roughly 9" handguard and gas tube
Rifle is roughly 12" handguard and gas tube

Now I have a ton of options on how to build this lower.  I now *think* I want a midlength gas system with a low profile gas block under rifle length free float tube.  Man that could be cool!  Or not.   Hell I cannot figure out what I want to do.  I already ordered the collapsible stock and am having second thoughts about just building an A2 style rifle.  So many variations, so little funding....

CMMG (in the industry section) has midlengths in 14.5" w/ perm. attached phantom and 16", as well as dissipators with carbine, midlength, or full length gas systems. I'm having the same dillema. I have one unused lower and too many options.

Proud owner of an all RRA middy I built myself.  It's an A4 fitted with ARMS SIR #58, ARMS #40 buis, TD vert grip, and Surefire weapon light.  I love it.  Like you said kind of a compromise between the two.  I find it more visually appealing too.


-Hershey