Sorry, "Middy's" for Dummies

[Cletus VanDam]

I truly apologize for another "Midlength" post. but up until a week ago, I had never heard the term "Middy". I am new to the site. I do not own an AR, yet, but I want one. I have read 42 pages of posts (alot touting Midlength is the way to go), done the search thing and read some more and the more I read, the more my head swells with info, but, I just don't get the Middy system and which AR's have it. Hard to visulize something you've never seen before.
It seems like every time someone posts about "Middy's" they link you to G&R Tactical's BCM M4 that is always "out of stock" (by the way) and there is nowhere in the description, that a novice can spot, that mentions "Midlength".
Are they the only ones who make a Middy? How do you tell?
I am interested in getting the latest tech in my first AR, and if midlength is the way to go, then that is what I want.
The three brands I am interested in are Colt, DD, & BCM.
Do Colt and DD have Middy's? If so, which ones?

Thanks for your time,
Cle

[USMC03]

This may help:

http://www.03designgroup.com/photo/carbine-vs-midlength/icon-carbine-vs-midlength.jpg
03designgroup | Carbine vs. Mid-Length Gas System on a 16" Barrel http://demigodllc.com/icon/extwh3.png (http://www.03designgroup.com/technotes/carbine-vs-mid-length-gas-system)


Colt doesn't make a Mid-Length. Last time I checked Daniel Defense's website they didn't offer a Mid-Length. They offer 16" barrels with Carbine length gas systems.


Bravo Company, Sabre Defense, Noveske, Rock River Arms, and several others offer 16" barrels with mid-length gas systems.

[Cletus VanDam]

AWESOME LINK USMC03!! THANK YOU!!All the info from the posts I've read are starting to sink in!!

[Cletus VanDam]

So, When someone takes a Colt 6920, 16" barrel, and changes it out to a 14.5" barrel, that "lengthens" the existing gas system 2" - so thereby making it a midlength gas system ?? on what was a carbine length gas system.?? Right??

Thanks,
Cle

[bkb0000]

the gas system consists of individual components.. some weapons, usually carbines, are configured with "carbine-length" gas system components.. when you change out these components to a "mid-length" system, you still have a carbine- but now your carbine has a mid-length gas system.

i think you're just getting terms confused.. easy to do at first.

[bkb0000]

i don't know what all you've read, but here's a quick in-a-nutshell that might help...

AR/M4 pattern weapons use "direct impingement" gas operation- as the bullet moves down the bore, it passes a "gas port" and continues out the muzzle. once the bullet passes the gas port, some of the gas propelling the bullet shoot up the port, follow along inside a metal tube that injects the hot, expanding gas into the action of the gun, cycling the action- ejects the spent casing and chambers a fresh load.

the AR/M4 pattern weapon system comes in various configurations and legnths. Weapons with 20" barrels and some 18" barrels use a "rifle-length" gas sustem- the gas port exists in the barrel at about 12.5" from the face of the receiver. some 18" barrels, 16" barrels, and 14" barrels use a "mid-length" gas system- which places the gas port at about 9.5" from the face of the receiver- the mid-length is shorter because the barrel is shorter, and it operates better this way (more on "dwell time" later). "Carbine-length" gas systems are used on some 16" barrels, 14" barrels, and basically all 12, 11, and 10" barrels- this system length is approximately 7.5" from the receiver.

The reason for the different gas port lengths is due to something called "dwell time." dwell time is the amount of time a bullet remains in the barrel after it passed the gas port- the longer the bullet keeps those gasses plugged up, the more pressurized the gas is that shoots down the gas system. there needs to exist a proper balance somewhere between "too much" gas and "not enough" gas pressure to correctly and reliably cycle the action.

carbine gas (7.5") generally creates a bit "too much" pressure in barrels longer than 14.5, and it can even be argued that it creates too much pressure even in 14.5s- thats why some 14.5s have mid-length gas (9.5). in 16" barrels, there's an additional 9-something inches of barrel beyond the gas port- this creates a HUGE amount of dwell-time (builds the pressure up a lot)- more so than basically any other barrel length/gas length combination. 16" barrels are by far the most common length in civilian weapons. this was a big problem for reliability and durability, as these weapons cycle really hard. "mid-length" (9.5) was created to reduce the dwell-time, and thereby soften the pressure of the gas, thereby softening the cycling of the action.

[JGou812]

So, When someone takes a Colt 6920, 16" barrel, and changes it out to a 14.5" barrel, that "lengthens" the existing gas system 2" - so thereby making it a midlength gas system ?? on what was a carbine length gas system.?? Right??

Thanks,
Cle

Not unless the gas tube is now longer.

The middy generally has a gas tube of approx. 9 inches. Midway between the Carbine (about 7") gas tube length, and Rifle (about 12") gas tube; hence the midlength name.

---
Or what bkb0000 posted.

[bkb0000]

Just so happens, i have three of the different gas systems dissassembled in my shop right now... disregard the mess, my work space is not usually that ****ed up.

from bottom to top- "carbine," "mid-length," "intermediate"- intermediate gas is only used by a couple of manufacturers, but i unfortunately don't have a "rifle" in shop to show.
http://c1.ac-images.myspacecdn.com/images02/130/l_417397b535ac423299d5f03b28cb699c.jpg
different angle- left to right- carbine, mid, intermediate
http://c3.ac-images.myspacecdn.com/images02/135/l_fd73c31fc51842a1865ac051dbbd48ca.jpg

these are two weapons that both have "carbine" gas, but are totally different barrel lengths.. 10.5" on top, 14.5" on bottom
http://c3.ac-images.myspacecdn.com/images02/119/l_3aad6cd7d7e14c70b916dac42004a88a.jpg

for some perspective.

[Cletus VanDam]

Thanks, bkb0000 & JGou812 for your responses. The side by side gas systems pics were great. So were your explanations. I am getting the full picture now.

Thanks again,
Cle

[ForTehNguyen]

midlength was created in response to the problems the carbine gas system was having: increased wear, higher pressures, high bolt velocity. To see how the midlength has advantages over the carbine, you have to understand what negative effects cutting the gas system from the original 20" rifle length gas system on the M16 to the M4 carbine gas system does:

Armalite did a study showing the performance differences:
http://www.armalite.com/images/Tech%20Notes/Tech%20Note%2048,%20Barrel%20Design,%20Heat,%20and%20Reliability,%20030824%E2%80%A6.pdf

I lot of these M4 "enhancements" (M4 feed ramps, higher tension extractor, stronger mag springs) were basically small fixes to address some of the problems the carbine had with it trying to feed too fast because of the higher bolt velocity.

the midlength helps correct the higher pressures and high bolt velocity the carbine experiences without increasing the barrel length. The midlength runs lower gas pressures (13k psi for rifle, ~17k psi midlength, 26k psi carbine), lower bolt velocity, reduced recoil (it is noticeable btw, i've shot both), increased sight radius, and IMO looks better. Basically the gas system was optimized around the original rifle gas length system but the carbine gas system changes that balance and causes some problems. The midlength just goes back toward the rifle gas system to help balance it out.

nothing wrong with a carbine, but when they are the same price, ill stick with a midlength. They have many advantages over a carbine. I have 4 ARs: 3 midlengths and 1 rifle

additional explanation here:
http://www.thefiringline.com/forums/showpost.php?p=3122760&postcount=4

[mrbieler]

Last time I checked Daniel Defense's website they didn't offer a Mid-Length. They offer 16" barrels with Carbine length gas systems.


DD evidently does have a midlength, but I'm not sure when it's going to his general consumption. Found this, but it's not on their regular web page:
http://www.danieldefense.com/?page=shop/detail&product_id=94