Hi Guys,

I'm in the middle of my first build here, scrounging for parts like the rest and I came accross a question about length of the gas system vs length of the stock's buffer tube. I've come accross two versions, comercial rifle length and the milspec telescoping carbine length buffer tubes. Are there other lengths? Which length is the A2 stock?

I was thinking if they were diffrent and I put a milspec telescoping stock (shorter, meant for carbine) on a lower and then put a full length upper on it, it might cycle diffrently. Do I have to match the upper gas system to the buffer tube length? What happens if I mount an M4 style carbine upper to a lower with a comercial rifle length buffer tube?

Final question. I bought a DPMS dedicated .22 LR upper with bull barrel (havn't received it yet) and I was wondering how the recoil system works in that, if it uses the buffer tube at all, it looks like it has it's own spring and area to recoil right in the receiver instead of the buffer tube. So would I even need a spring or buffer in a lower that I use only with the dedicated .22 upper?


Thanks guys, I'm learning so much from this site every day it's insane.

You're actually asking two different questions.

A rifle buffer system is used with a fixed stock, like the A2 or Magpul PRS.

Collapsible stock buffer tubes are shorter, and as such require a different spring and buffer.
They are further broken down into "commercial" and "milspec."
That has nothing to do with the length of the tube.
Commercial carbine buffer tubes have an outside diameter of 1.17"
Milspec carbine buffer tubes have an outside diameter of 1.14"

Now, as to the gas system, no, usually you do not have to match them up.
You can run a fixed A-2 stock with a short barrel, and it works.
You can run a carbine stock and buffer on a 20" rifle gas system and it works. This is how my gun is currently set up.

Generally, guys will be running collapsing stocks with shorter barrels to make a more compact package, as that's what they wanted from the beginning with the short barrel.

Usually means that if you go to a really short barrel, you may have to play around with the weight of your buffer (H, H2, H3) depending on your gas port diameter and wear.

Gas system length and receiver extension length are completely unrelated.  

In general there are 3 gas systems (short to long)
Carbine
Midlength
Rifle

There are 2 receiver extensions/stocks
Carbine
Rifle

The Carbine length extension is available in mil-spec or commercial diameter but are the same length.
There is no such thing as a commercial rifle receiver extension.  They're all the same size.
The Carbine length extension is for the collapsible stocks (also entry length fixed).
The Rifle extension is an A1 extension.  The A2 uses the same extension with a spacer.

In a Carbine length receiver extension, the weight of the buffer should be matched to the length of the gas system.  General options CAR, H, H1, H2, 9mm, there are others.
All Rifle length buffers are the same and will work with all gas systems.

I have no experience with the 22LR uppers, but I've seen pics of them with folding stocks, which tells me they recoil inside the upper receiver and don;t need the receiver extension.

ETA
Rifle- The weapon was designed to use this gas system length. THe length of the gas tube is 10 and 3/16ths inches. It has the softest recoil of any standard direct impingement gas system length. There is less wear on the bolt carrier group parts. You get the 20 inch full rifle length barrel which gives you more muzzle velocity.  Also used on the 18" SPR.

Carbine- This system was designed next. The gas tube is 7.5 inches long. You get the handiness of a carbine with a barrel length of 16 inches or 14.5 inches with a muzzle device that brings it out to 16 inches. You can get an SBR from 14.5 to 7.5 inches. This system is more "violent" than the rifle system. The problems of the carbine gas system have been addressed by heavier buffers and extractor upgrades and these updates have improved this system tremendously.

Midlength- Relatively new compared with the other two. It was designed to remove some of the harshness of the carbine system, keep a 16" carbine length barrel and to lengthen the sight radius for improved accuracy. This gas system is approximately 9 inches. The midlength is a civilian creation. Nothing negative or positive meant by the last sentence, only a small fact.

Wow, thanks for clearing all that up for me. Those are both great explainations for a subject that has been unclear to me. I'm trying to go through my build one part at a time and figure out whats going to work for me so I hopefully won't end up with more parts than I need or the wrong stuff.

What I have now is a complete carbine setup, so if I wanted to go with an upper with a rifle length gas system I should then change out the buffer to the appropriet weight? I"m guessing H1 is carbine, H2 is midlength and H3 is for rifle length gas systems? What would happen if I installed a H3 buffer and then tried to use a carbine upper? Would the bolt then short stroke and not feed additional rounds? Or do I have it backwards and the carbine buffers are heavier?

From what I understand, the Navy came up with the different weight buffers to tune their Mk18 rifles.
These rifles have 10.5" barrels.

For a regular 16" carbine, the buffer isn't as critical. Even less so if it is a mid-length gas system.
I run an H buffer in my gun (rifle gas/carbine stock) and it works just fine.

All of this below applies to carbine length extensions.

The order I listed were from lightest to heaviest.
The rifle length gas is the "softest" so will work well with the lighter buffers.
The carbine gas is the "harshest" so it needs a heavier buffer for optimum service life.

The shorter gas systems need the heavier buffers.
The distance from the gas hole in the barrel back to the action is shorter.  
So it takes less time for the gas to reach the action.  
So the pressures are higher and the recoil more "violent".  
The action unlocks more abruptly, things fly around faster.
A heavier buffer keeps the action locked longer and lets the pressure subside slightly.

The original design is the rifle length gas system.  
By the time the bullet gets to the gas hole the pressure "spike" is much lower and the slope of the pressure curve flatter.
If you put an H3 buffer in a weapon with rifle gas, you may get short stroking.  
The rifle buffer weighs the same as an H3.  The Carbine spring, however is stiffer.  
So the combo of stiffer spring and heavy buffer may prevent reliable cycling.

This is the reason midlength was developed.  
If you are limited to 16" barrel (civilian minimum without NFA stamp), then you can benefit from the longer gas system on the same size barrel.

A rifle gas system would work fine with a CAR buffer in a carbine length extension.  

In general a midlength and rifle length can use the lighter CAR buffer with no problem.  
Most carbine length rifles also come with CAR weight buffers from the factory.  
But the action is violent and will be prone to damage overtime.  The Navy has a very good report on this for their Mk18s.

The military M4s use H or H2 buffers.  H for the M4s and H2 for the full auto M4A1 IIRC.  And the H3 is for the ~10" barreled guns.

Some people even use the heavy 9mm buffer with good results.

It's fairly easy to open up the buffer and trade out weights inside to experiment.
An H3 has 3 tungsten weights inside (not pictured below).


For me.
I use H2 buffers in my carbine gas weapons and H in my midlengths.


Slash has a good table of the buffers and weights


AR-15 Barrels pic of inside construction.