Mid length & M4 Carbine

[Archangel14]

I was perusing the BCM web site and I see that they sell complete upper barreled groups in "Mid Length" and "M4 Carbine". Did some research to determine what the difference is, but now I'm just thoroughly confused. So, I now turn to my buddies at THR. Educate me please, what's the difference?

 

[Skylerbone]

Three gas lengths, carbine, mid-length and rifle. Meant to match barrel length to gas pressure to prevent over gassing. Probably a few YouTube videos that could point out the benefits/applications of each.

 

[jbj]

The longer the gas tube, the longer the dwell time of the gas in the tube. The longer the dwell time of the gas in the tube, the less sharp the recoil impulse (the recoil energy curve flattens out over time).

 

[jr_roosa]

Basically you have a choice of barrel lengths and a choice of how far down the barrel the gas port is.

An M4 has a 14.5" barrel and a "carbine length" gas port.

For NFA rule issues, most civilian carbine style M4geries have 16" barrels. The "mid length" gas port is further down the barrel, but is just as far from the muzzle of a 16" barrel as the "carbine" port is from muzzle of a 14.5" barrel. Also, with a mid length gas system on at 16" barrel, the bayonet will work properly and the front end of the rifle "looks right" when compared to an M16 or M4.

You can mix and match all day long, but the further from the muzzle you put the gas port, the more gas and energy are harnessed for the gas system. You want a "Goldilocks" level of gas. Not too much, not too little, although you may have no problems at all on a particular rifle that may fall outside of the ideal.

A carbine length gas system on a 16" barrel is said to be "overgassed" even though there are millions of them out there working pretty much OK. Really short barrels on a carbine can be "undergassed."

People tweak the buffer weights and other factors to deal with this issue.

-J.

 

[MistWolf]

The longer the gas tube, the longer the dwell time of the gas in the tube. The longer the dwell time of the gas in the tube, the less sharp the recoil impulse (the recoil energy curve flattens out over time).

What is important is the distance from chamber to gas port. The further the distance down the bore, the less pressure there is to tap into at that point.

Port diameter is also important. The larger the gas port diameter, the more volume of gas that will flow through it, depending on pressure.

Another important factor is blow down time. Blow down time is how long it takes the gas system to blow down to ambient pressure after the bullet has uncorked the muzzle. This is important because, for a variety of reasons, the gas does not charge the system and start the BCG moving until after the bullet has cleared the muzzle. The rifle is actually operated by the residual pressure. If the pressure drops too soon because the port is too close to the muzzle, the rifle will be unreliable even with a large gas port. If the distance is too far from the muzzle, a smaller gas port may need to be used.

If an AR carbine is tuned properly, there is no real practical difference between a 16 inch barrel using a middy system or carbine system. My personal preference is to use a mid-length gas system with the 16 inch barrel and a carbine length gas system with a 10.5 inch barrel

 

[Tomac]

I prefer midlength over carbine, I like the recoil impulse better and the longer handguard. BCM's lightweight barrels are nice, about 5oz lighter than standard-weight barrels. In fact, I recently purchased 3 of their lightweight 16" midlength uppers w/free BCGs & Mod 0 comps for $439/ea, great deal!
Tomac

 

[Bartholomew Roberts]

My rule of thumb:
10.5"-14" - Carbine gas
14.5"-16.5" - Midlength gas
17"+ - Rifle gas

It is usually advantageous to go with the longer gas system in 5.56. You get a better sight radius. You have more handguard real estate and protection for the barrel, and the further the gas port is away from the chamber, the slower the gas port is eroded by unburnt powder.

The flip side is you can't have the gas port too close to the muzzle for reasons already explained. You want about the same distance as on the original 20" AR15 for function; but you can tweak that with different port sizes.

 

[MachIVshooter]

My rule of thumb:
10.5"-14" - Carbine gas
14.5"-16.5" - Midlength gas
17"+ - Rifle gas

Pretty close to my own. My preferences are:

<11"-pistol gas
11"-14.7"-carbine gas
16"-18"-mid length gas
>18"-rifle gas

A larger gas port or a muzzle device like the Noveske KX3/KX5will compensate for a shorter dwell time, which is the case when you have <4" of barrel forward of the gas port. A smaller port is sometimes used when dwell time will be excessive, as is the case with 16" barrels using carbine gas systems, 18" mid lengths or 22+" barrels.

A buddy of mine got a middy barrel that was drilled like a 16" carbine tube, and we ended up having to open the port 0.015 to make it run right. Conversely, we've had some 16" carbine barrels that were drilled too large, and were still violent even with a heavier buffer.

Bottom line is, while you can adjust port size and buffer weight to deal with over or under gassed designs, it is best to have dwell time as close to ideal as possible for smooth and reliable operation.

 

[carbine85]

The mid length in the carbine is the way to go. The felt recoil is less and you can achieve quicker follow-up shots. I have both and prefer the mid length with the full auto carrier or H1 buffer but not both.

 

[Archangel14]

Thank you guys.

 

[Doogledog]

I have a BCM midlength 16" and 14.5" that I run a full auto carrier and and H1 buffer in and I really like them both. I had a carbine length 16" that I got rid of because the midlength felt so much smoother to me. My 14.5" has a permanently pinned and welded mod 1 comp so I don't need a stamp but as others have stated I wouldn't have a carbine length unless it's shorter than 14.5".

 

[J_McLeod]

I have two ARs with 16" barrels. One is mid length and the other is carbine length. I like the mid length better and think it has slightly less recoil and muzzle climb.

 

[dprice3844444]

http://www.brownells.com/rifle-part...ubes/ar-15-m16-pigtail-gas-tube-prod1591.aspx

want a midlength gas system feel with a carbine barrel?

 

[eldon519]

Carbines are also said to be harder on parts such as extractors and bolts when compared to a midlength or rifle gas system.

 

[gotigers]

Bartholomew Roberts

My rule of thumb:
10.5"-14" - Carbine gas
14.5"-16.5" - Midlength gas
17"+ - Rifle gas

It is usually advantageous to go with the longer gas system in 5.56. You get a better sight radius. You have more handguard real estate and protection for the barrel, and the further the gas port is away from the chamber, the slower the gas port is eroded by unburnt powder.

The flip side is you can't have the gas port too close to the muzzle for reasons already explained. You want about the same distance as on the original 20" AR15 for function; but you can tweak that with different port sizes.

BR nailed it.

My favorite gas system for he M4 or carbine AR15 is midlength.

 

[M1key]

My favorite...is an A-1 butt stock assembly for carbine lengths. I like the idea of the pigtail gas tube, too.

American ingenuity.

M

 

[jad0110]

I prefer midlength over carbine, I like the recoil impulse better and the longer handguard. BCM's lightweight barrels are nice, about 5oz lighter than standard-weight barrels.

I agree, I also have a BCM 16" mid length pencil barrel upper on my AR. Nothing fancy, which is exactly what I want in a home defense carbine. Quick handling over all out accuracy is what I prefer in that case.

Unless you are going with an AR pistol or willing to pay a $200+ tax stamp (don't recall what it is these days), a 16" mid length is a nice, well rounded setup.

I have since changed out the pistol grip and hand guards and added a light, but the balance is still pretty decent with the lightweight barrel. I used to have a HBAR Del Ton middy upper on this rifle. Combined with the light weight of the adjustable stock and the light (had I mounted it back then), it would have felt terribly muzzle heavy IMO.

 

[jabram]

Does the Colt factory 6920 have a carbine length gas system? I've been thinking about purchasing one since prices have came back to earth.

 

[M1key]

Does the Colt factory 6920 have a carbine length gas system? I've been thinking about purchasing one since prices have came back to earth.

yes...

and an H buffer

M

 

[B!ngoFuelUSN]

Pretty close to my own. My preferences are:

<11"-pistol gas
11"-14.7"-carbine gas
16"-18"-mid length gas
>18"-rifle gas

A larger gas port or a muzzle device like the Noveske KX3/KX5will compensate for a shorter dwell time, which is the case when you have <4" of barrel forward of the gas port. A smaller port is sometimes used when dwell time will be excessive, as is the case with 16" barrels using carbine gas systems, 18" mid lengths or 22+" barrels.

A buddy of mine got a middy barrel that was drilled like a 16" carbine tube, and we ended up having to open the port 0.015 to make it run right. Conversely, we've had some 16" carbine barrels that were drilled too large, and were still violent even with a heavier buffer.

Bottom line is, while you can adjust port size and buffer weight to deal with over or under gassed designs, it is best to have dwell time as close to ideal as possible for smooth and reliable operation.

This is really interesting stuff. The details are new learning for me. Just to probe:
1. Does the optimization of dwell time vary between a semi-auto and an auto assuming all other parameters are the same?
2. Why doesn't the AR world adopt adjustable gas ports like some other designs employ? Is it just added complexity? More complicated cleaning? Just didn't think it was necessary?
Thx,
T

 

[Downeast]

Thanks to Archangel for asking this and for all the detailed responses. I now have a greater understanding of something that previously confused me.

 

[carbine85]

This is really interesting stuff. The details are new learning for me. Just to probe:
1. Does the optimization of dwell time vary between a semi-auto and an auto assuming all other parameters are the same?
2. Why doesn't the AR world adopt adjustable gas ports like some other designs employ? Is it just added complexity? More complicated cleaning? Just didn't think it was necessary?
Thx,
T

1) Dwell time only changes with the length of the gas system.
2) I suppose it isn't necessary in the DI system. It is used in the piston guns.

 

[MachIVshooter]

1. Does the optimization of dwell time vary between a semi-auto and an auto assuming all other parameters are the same?

As Carbine85 mentioned, dwell time is a function of gas system length-specifically, how far the port is from the muzzle. Semi or full auto capability is irrelevant.

2. Why doesn't the AR world adopt adjustable gas ports like some other designs employ? Is it just added complexity? More complicated cleaning? Just didn't think it was necessary?

They are available, and some people like them for suppressed use, where dwell time is increased substantially by the suppressor, which adds a tremendous amount of volume between the port and muzzle.

For unsuppressed shooting, though, the Stoner gas system is highly adaptive. In my mid lengths, I've had no trouble with everything from TULA (well known to be underpowered) to some really warm handloads spitting 40 grain bullets out at over 3,500 FPS from the 16" barrel.