Ar 15 gas tube help needed

[nekshot]

I do not understand the gas tube length and why. I am ignorant of this info so you tell me in simple terms what the deal is with different lengths and hopefully I will be more informed! I do know what the tube does but what determines a rifle length tube just be cause that is what I want with my x39 which is now I believe at carbine length. The tube is now approx 9 inches.
Have fun and a merry Christmas, nekshot

[KMac]

232588

[Kylongrifle32]

If you are building a rifle there are actually 3 lenghts to choose from. Carbine 6.75" / mid lenght 8.5" / rifle 12". This is determined by where the gas port hole in the barrel is located. The gas block is attached to the barrel over the gas port. The choose of gas tube lenght is a matter of how far the gas travels back to the bolt to cycle the next round. This is not as critical in a civilian AR-15 but in a full auto M-4 or M-16 the shorter the travel distance the faster the cycle rate of fire is obtained. That said it only depends on you choose of hand gaurd you are using. If you use the clamp on style Delta ring hand gaurd you are limited to the length of hand gaurd ie. Carbine 6.75 - rifle 12 inch. If you are using a free-float style hand gaurd with a low profile gas block you can use a carbine gas system with a 15 inch hand gaurd over it.

Hope this answers your question if not shot me a PM are I can go into more detail

[Moleman-]

It's hard to put into a single post, but pressure-volume-time. You need the right balance of these three things in order to cycle the action smoothly. If you're a little short on one you can fudge it a little bit by increasing the other two, but only to a point before issues crop up or it no longer will work. Generally 12K to 14K psi at the gas port is what you want. Most of the charts you'll see on ar15 gas systems are biased off of 223/5.56 so look at a similar cartridge and see what gas tube they are using on it and why or look at a pressure per inch graph for the cartridge you want to use. Most of the 16"-20" 7.62x39 barrels you'll see are carbine gas with a few at midlength. By the time you get out to 12" the port pressure would be around 9Kpsi which would likely start to mean lighter loads may not work even if you opened up the gas port to .116" which is around the ID of a gas tube (they vary in ID). The closer you get the port diameter to the gas tube diameter there is a diminishing return as just so much gas is going to get down the tube in a given time at a certain pressure. Working with wildcats a lot I'll look at a pressure per inch graph of the cartridge and see where the pressure is around 12K psi and put the port there. If I want a longer handguard I'll generally put a free floating handguard over the gas block, but you can also add a dummy gas block out further to hold a set of handguards on.

[nekshot]

Ok, this helps but another question is how does all this relate to the buffer tube length? Once I get this all straightened out in my head I will need to take a long coffee break!

[Gtek]

It only relates into what stock set up YOU want underneath it. Adjustable stock receiver extension/buffer tube gets a carbine buffer and spring. Rifle extension uses one tube (A1), added spacer to (A2) length, or aftermarket rifle usually has a rifle buffer and spring. Correctly set up uppers will usually work fine on either lower configuration. The carbine has some play room with different weights available in buffers to tweak BCG reaction, pistol usually is a carbine length set up. Coffee? I know I said USUALLY a lot but maybe a 24 pack of Mountain Dew will be in order here for this rabbit hole!

[nicholst55]

Ok, this helps but another question is how does all this relate to the buffer tube length? Once I get this all straightened out in my head I will need to take a long coffee break!

Now you REALLY opened a can of worms! Not just buffer tube length, but buffer weight! Let me try to muddy the waters. Normally, carbines with adjustable length buttstocks use the short, carbine-length tube. You can use a fixed buttstock and a full-length tube, like was done during the 1994-2004 federal Assault Weapons Ban. Rifles (with fixed buttstocks) use the full-length tube. With the carbine-length tube, you need to use a short carbine buffer. With a rifle, the long rifle buffer.

When using a rifle with an adjustable stock, you NORMALLY use a carbine-length tube AND buffer. Some proprietary rifle adjustable stocks come with a rifle-length tube and buffer. If they do, use the parts that the manufacturer provides. There are exceptions to these rules-of-thumb, but these are generally correct. I won't go into buffer weights, because there are too many exceptions.

Some really long rifle barrels use proprietary, extra-long gas tubes, but the manufacturer normally specifies that and sells them.

[nekshot]

I really appreciate all your help. I am getting the drift of it but I need to get it all clear in my head. This all got spinning me outa control when I got the idea to put a wooden forend all the way to the end of the 16 inch barrel. To hold the stock I thought I will use a gas tube with a little rail on it (for the sight if I want it) out to an inch of barrel. The barrel now has a 9 inch gas tube but I put a sleeve over it. My meds are screwing with my hands losing feeling. I cant do much wood work put I can push buttons as in the lathe and such. So far we have gorgeous walnut wood on the ar stock and pistol grip. A little more up front would really look nice.