I am going to buy one AR upper and rig it with: 4-rail handguard (don't know which one yet), flip up front and rear iron sights, M93 buttstock and some other stuff like grip etc.

I am not sure which way to go. M4 or Mid-length carbine? I am inclined towards a 16" barrel. Which are the pro's and con's of the M4 type versus Mid-length carbines?

Quoted: I am going to buy one AR upper and rig it with: 4-rail handguard (don't know which one yet), flip up front and rear iron sights, M93 buttstock and some other stuff like grip etc. I am not sure which way to go. M4 or Mid-length carbine? I am inclined towards a 16" barrel. Which are the pro's and con's of the M4 type versus Mid-length carbines?

The midlength gas system barrels are also 16 inches in length, but they are supposed to be more reliable in function than the regular short gas systems.

Less Port Pressure, reducing the violence and speed of the cycling plus less likely to cause feed problems due to bolt cycling faster than the magazine can raise a cartridge.

Slower cycling, means extractor is less likely to jump off cartridge rim, due to centrifical force of bolt unlocking to fast.

Later timing on opening the bolt, allowing chamber pressure to drop more.

Dwell time, after the bullet passes the port and before it leaves the barrel, duplicates the 20 inch rifles.

More balanced appearance, and more "real estate" on the forend.

Ability to mount a bayonet properly, should you desire.

To me, it looks like a win/win situation, in practicality.

If you HAVE to have the "M-4 Look," simply have someone turn down the barrel in the appropriate place, to mount your M203.



Lonny

Do all midlength uppers come with a longer gas tube when compared to M4 types?

Not certain, but I believe that whenthey refer to a "midlength" barrel, I BELIEVE they are referring to the gas tube length.



Lonny

Quoted: Do all midlength uppers come with a longer gas tube when compared to M4 types?

Yes, it's about two inches longer.  Put another way, the distance from the gas port to the muzzle is the same as on a 20" rifle.

Yes, it's about two inches longer. Put another way, the distance from the gas port to the muzzle is the same as on a 20" rifle.

Wait, thats a dissipator you are talking about right?

Oh, no I get it, from port to muzzle... that makes sense.

snipped from another post:

People have been moving the gas port around on the barrel for as long as the rifle has been around, but here is a quick run down -- some of this is my opinion and I know there are people in the industry that do not agree with all of it, but have not gotten a good argument against it yet.

The original rifle uses a 20" barrel with the gas port at 13" (these figures are *close enough*), during the development of the XM177/Commando/Carbine the barrel was cut to lengths from a little more than 10" to 11.5" for these rifles, the gas port was moved to 7.5" -- as you can see from the chart on the Armamlite page, the port pressure at 13" is around 13,000 psi and nearly double that at the 7.5" position. What is a more important measurement really, is the peak pressure inside of the bolt carrier, which is again nearly doubled for the short (carbine) gas system v. the long (rifle) system.

The pressure at the port is not the only thing in play here, the amount of time (dwell) that pressure exists is a major factor... the original rifle design has 7" of barrel past the gas port to determine dwell -- along comes the M4... in a quest for a shorter barrel (the XM177 project was determined unreliable), here is were that 7" of barrel comes in to play again, but this time for a different reason entirely.

That 7" is also what is needed to mount a bayonette to the standard FSB lug and flash suppressor, Colt already had the tooling and production for a rifle with a gas port at 7.5" (from the carbine)... so, if you add 7" to that (for the bayonette), well guess what the barrel length would be?

That part is my mind at work, I am told that the rifle was specced with a 14.5" barrel and it all happened to just work out really well...

Now, here is one of the first problems, with the increased gas pressure at the carbine length gas port, the dwell time becomes long and the rifle is trying to unlock early... bolt velocities are also way up there (pressure inside the bolt carrier are now very high). I prototyped a rifel at the SHOT Show last year that some may have seen, that uses a mid-length gas port on a standard M4 barrel (14.5), mounts a bayonette and GL and still uses carbine handguards and all the doo-dads.

The mid-length system moves the port to 9.5" with 6.5" of barrel (dwell) past the port... this system works so well with a 16" barrel it is not funny.

We made some dissy stlye uppers using 16" barrel and mid-length gas system, they were very nice little field carbines, the only downside was that you can not mount a bayo or telescoping suppressor to the gun... the alternitive to moving the gas port position is to make it smaller or larger so the gun fucntions -- Very few shops will bother to take pressure readings (or even do computer genterated models) of what the operating pressure of the rifle is in a particular setup. The internal bolt pressure should run 1.5 Kpsi -- For you gear heads think of it like this, GM builds a motor that is designed to run with a 8:1 compression ratio, then someone comes along and stuffs a supercharger on it without making any other mods, sure it will still run, just not quite right (and probably not near as long). Moving the gas port around is a bit of work granted, but i maintain that it is the best way to get things running right.

So, you got the 3 common lengths, carbine - mid - rifle, that have been accepted in the industry. We deal with 2 other locations for specialized rifles as well... anyway, hope it helped someone?

Somebody at Armalite write that?

Quoted: Yes, it's about two inches longer. Put another way, the distance from the gas port to the muzzle is the same as on a 20" rifle. Wait, thats a dissipator you are talking about right? I am talking about midlength... are they the same???

No, standard Dissipator type rifles use CARBINE length gas tubes under FULL length handguards.

HOWEVER, as Gunzilla posted it is possible to make one with a mid-length gas tube.  I guess that makes too much sense since no manufacturer mass produces one...

So where to get mid-length uppers??

Quoted: Somebody at Armalite write that?

I almost fell asleep halfway through reading it so it sure seems like something ArmaLite would write....

Quoted: Somebody at Armalite write that?

No... I did

It is from another thread I answered, I have *nothing* to do with Armalite

I can put in pictures and graphs, but it gets even more boring...

Quoted: So where to get mid-length uppers??

ADCO built mine (RRA parts) and it freakin' rocks.  I entered two carbine matches with it and won both.  This brings up another plus.  The extra sight radius is a big advantage at longer ranges when competing against others with iron sights.

Quoted: It is from another thread I answered, I have *nothing* to do with Armalite I can put in pictures and graphs, but it gets even more boring...

Sorry dude, I just couldn't resist messin' with you....

My two.

16" M4 barrel on top and 16" midlength (gov't profile under handguards) on the bottom. The mid is 4 ounces heavier than the M4 but I can't tell you where the weight comes from because I haven't weighed individual compnents. The lowers are identical with H buffers and they both have the same carriers and upper receivers. The mid has a slightly different recoil pulse. Feels like a slower push than the M4.

I almost forgot:

If you take a look at ADCO, consider the contoured profile option that IB mentioned.  It takes off about 12 ounces of weight and tremendously improves the balance.  I think it was well worth the additional expense.

Personally I find the midlength carbine looks more balanced in terms of the length of the barrel in front of the front sight.   I find the really long barrel out in front looks disfunctional

I like the longer gas system, longer sight radius and more balanced look personally.

Jeff

I have seen RRA and armalite mid-lengths.... anyone else makes them? How about Bushmaster?

Yes!!!! Something like that. The DD 9.0 looky like what I want. I gotta talk to the folks at ADCO. What manufacturere upper is that?

Quoted: Quoted: So where to get mid-length uppers?? ADCO built mine (RRA parts) and it freakin' rocks.  I entered two carbine matches with it and won both.  This brings up another plus.  The extra sight radius is a big advantage at longer ranges when competing against others with iron sights.

Id like to see the pics and graphs. more of a visual learner