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GA, USA
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Posted: 5/26/2006 10:40:34 AM EDT
[Last Edit: engineer2001]
[#1]
Hi. I'm a noob (sorry in advance). I'm looking to buy a new 16" upper for my next build, and I have been directed to this thread after asking about reliability and gas pressures. I don't want my carbine wearing out if it's avoidable, and all of the mid-length literature I have read says the standard carbine will wear out faster. I know marketing plays a factor as in, "we have a new product - time to trash the old regime to get it to sell," so I don't know how much of that is the case here.
In light of these pressure readings in the chart on page 1 of this thread, does anyone know how the higher pressure of carbines versus mid-lengths will attribute to wear of the gas system and the moving portions of the weapon? Just how much more "smooth and longer-lived" is a 16" mid-length setup if maintained properly versus a 16" standard carbine? Should I even worry about getting a mid-length gas system if all I intend to do is shoot in a range-based semi-auto fashion for thousands upon thousands of rounds over years of use and properly maintain the weapon? Do smoothness of cycle and lower pressures only really matter if I use them for full-auto or abuse my weapon (mag dumps, bump-firing, and such stupidity)? Thank you in advance! ETA: I refer above to .223/5.56 caliber weapons. Sorry for all the questions - if any at all get anwered, I'll be happy. 
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Posted: 5/26/2006 11:14:58 AM EDT
[#2]
Fear not, the US military has 1000's of carbine length M4's in use. If you have the choice, go mid-length, but don't choose mid-length just because everyone says you have to. Get what you want, take care of it properly and if it's quality kit, it will serve you just fine. Now my personal 3 gun match rifle is an 18" barrel with rifle-length gas system and a big brake on the end. You want to talk about smooth shooting... It will almost keep a penny sitting on the top of the barrel without falling off between shots. |
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Posted: 5/26/2006 11:27:36 AM EDT
[Last Edit: txgp17]
[#3]
One of the most informative posts I've read since joining this site.
Thanx bunches |
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GA, USA
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Posted: 5/26/2006 11:33:48 AM EDT
[#4]
That's what I wanted to hear. Thank you! |
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Posted: 6/10/2006 11:32:42 PM EDT
[Last Edit: MrMojoRising]
[#5]
The link below is to an old training video produced by the US .MIL. It's extremely dated but the basics and the how and why are still the same. Plus it's pretty damn entertaining.
www.archive.org/details/Rifle556mmXM16E1OperationandCycleofFunctioningTF93663 |
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CA, USA
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Posted: 9/12/2006 2:04:07 AM EDT
[#6]
A big thank you to Randall Rausch for posting all of the info on the AR's gas operation! It has given me some insight on a problematic AR-15. I now know it was having a short-stroking problem, but I didn't know it was called that at the time. Some modifications were made to a "plain jane" A3: YHM handguards, flip front and rear BUISs and a 6 position collapsible stock. I had told my friend that perhaps something's wrong with the buffer spring in the stock, but now I'm not so sure. Could it simply be that my friend didn't tighten the the gas block of the flip front sight? I don't recall the front sight being loose, however...
 Should the gas block be checked out, or should the fixed stock (and everything else connected to it) be reinstalled? |
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Posted: 9/12/2006 2:14:07 AM EDT
[#7]
Yes, it could be as simple as the gas block not fitting right, or not being aligned to the gas port. Those YHM flip-up front sights need to be forward about 1/32" from the shoulder on the barrel. |
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CA, USA
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Posted: 9/14/2006 1:42:18 AM EDT
[#8]
 Now he just has to find the time to go to the range and hope he doesn't have to pull the charging handle for each round he fires. 
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Posted: 9/14/2006 10:54:37 AM EDT
[Last Edit: Dano523]
[#9]
The key has more than enough room in its channel to accommodate the added length of a Gas tube/ block pushed all the way against the barrel shoulder. You have to remember that the differences in room we are talking about are only about 1/32" (the thickness of a front hand guard cap).
Also, just tightening up the gas Block screws may not be enough if the gas block channel to barrel port cannel is not correctly aligned. What you may suggest to your friend is that he pull the gas block forward of the barrel gas port, make a center line on the barrel shoulder dead center of the gas port, and use this line to reinstall the block (use the center line of the gas tube in the block to mate with the penciled line). Much too often, the barrel gas port may be off index with the upper's center line, and if he just uses the upper center line alone, it could be the problem. Granted that there is a little give room of the channels matching, but if the barrels gas port is not even close to the upper's center line, he may be looking at having to reinstall the barrel to get the correct zero and a functioning rifle. |
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MT, USA
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Posted: 9/21/2006 12:57:21 AM EDT
[#10]
I would like to say thanks to all the people on this thread who have helped me fix my problem. This is my first post so I didn't even have to ask, there is enough info right here for me to figure out my problem and fix it in house. That means I didn't have to send my upper back and wait how-ever-many weeks to get it back so I could shoot the flippin' thing. I just bought my first AR about a month ago, a Bushie 11.5" bbl. I read the probs with shorty's and was afraid I got one of those. There are 3 of us here who bought shorty's (another 11.5" and a 10.5") and they have had no probs. I, on the other hand, thought I bought the shortest single shot in the state. My problem was a short stroke and through this thread I learned how the gas system works and have a better understanding of the gun. I thought I would have it fixed when I learned I needed an "H" buffer. I got one and still had the same trouble. When I started really looking at my gun, after reading this thread, I noticed a "powder shadow" on the gas tube in the gas block. This was my problem. The gas tube and the gas block were not lining up correctly. Why? No roll pin. The roll pin that holds the gas tube in place had not been installed and the gas tube travelled forward and misalligned on the block. Bushmaster is a top notch company and I'm sure this one just fell through the cracks, I just got lucky and bought the one the quality assurance (QA) inspector pencilwhipped, lucky me. Anyway..... Thanks guys, I learned a lot from this site and will keep coming back.
When I get my Gemtech Halo I'll post a pic. Thanks. htVF-21 Freelancers 1989-1992 |
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Posted: 9/22/2006 9:13:39 PM EDT
[#11]
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Posted: 9/30/2006 1:09:09 AM EDT
[#12]
I've been trying to figure out what all that gas was doing in my rifle's bolt and this is the best information that I've been able to find anywhere.
During my search, I've also been tearing apart my AR and trying to see how the parts interacted. I have not seen anything in this thread about the firing pin. It has a flange that protrudes substantially above the bolt and, it seems to me must catch a substantial part of the gas pressure pushing backward. This would extract the firing pin from the bolt face and I was wondering what part this plays in function? |
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Posted: 9/30/2006 2:52:48 AM EDT
[#13]
The stop collar on the Firing pin is part of a safety device/ protrusion limiting-factor of the FP out of the bolt face, and never during ignition, a sealing factor.
When the bolt is cam'd/lock home, the back of the bolt protrudes out the back of the carrier, which allows the FP to max out against the back of the bolt/allows full bolt face protrusion. Now if the bolt is not locked home, then the back of the bolt is contained within the carrier, and even if the hammer does strike the FP, the FP will not achieve full protrusion out the bolt face. |
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CO, USA
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Posted: 10/10/2006 11:33:27 PM EDT
[#14]
Gas tube mushroom & alignment questions:
What exactly is the mushroom? On my new sabre AR, I hear a scraping as the bolt carrier slides the last bit forward. I am thinking it's the gas tube touching one side of the key interior as the key slides over it rather than going dead center into the key. Is this a misalignment problem, or of little concern. The rifle runs fine, the scraping noise just got me wondering. Thanks! |
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Posted: 10/11/2006 6:06:56 AM EDT
[#15]
Pull the bolt from the carrier, then walk the carrier into the locked postion.
As long as the end of the gas tube enters the key cleanly, and the sides of the tube is not scapping down the side of the key when the carrier face kisses the barrel extenion face, you're golden. |
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Posted: 10/11/2006 9:40:08 AM EDT
[#16]
It's the "uncurcumsized" end of the gas tube. Look at the last 1/8" of the tube in this picture: 
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CO, USA
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Posted: 10/11/2006 9:50:43 AM EDT
[Last Edit: 2ndChildhood]
[#17]
Thanks!
That's what I thought (Mushroom). Now how much misalignment is tolerable? (how to quantify? how to fix?) I'll try Dano's suggestion & see how it sounds. |
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Posted: 10/11/2006 2:32:20 PM EDT
[#18]
The end of the tube should enter the key cleanly. If needed, pull the hand guards and tweak the tube over the top of the barrel to get the end of the tube aligned with the carrier key.
Now if the tip of the gas tube enters the key fine, but as the carrier starts to touch the barrel extension, the key rubs the side of the tube after the mushroom end, then you will need to tweak the end of the gas tube protruding inside the upper receiver to get it straight, then go back and tweak the tube about the barrel to realign. As long as the barrel nut was correctly installed (read the tube can move around in the upper a bit, and not bound up on one side of the barrel nut spines), the key should have some movement to self correct after the initial mate with the key. |
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CO, USA
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Posted: 10/11/2006 10:32:32 PM EDT
[#19]
Thanks Dano & Randall!
It definitely was the mushroom rubbing as it enters the mouth of the key. I could only work inside the receiver as I have a floated handguard & no tools to remove it. The barrel nut does allow some movement of the tube so that is good. After I carefully tried to bend the tube I got the scraping reduced a bunch, but not completely. It's just scraping the mushroom, not the side of the tube. I think I'll live with it as-is for now unless you think it's a mistake to do so. Is it correct that my risk right now is that it will wear down the mushroom over the long term? Maybe one of the local guys can bring tools to a shoot sometime & I can try to get it perfect. |
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Posted: 12/24/2006 3:56:26 AM EDT
[#20]
Great Job Randall!
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USA
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Posted: 1/1/2007 10:07:23 PM EDT
[#21]
The gas tube I got from Bravo doesn't have a mushroom, it's straight.
JFA |
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Posted: 1/1/2007 11:12:37 PM EDT
[#22]
Something's wrong... Post a close-up picture here. |
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USA
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Posted: 1/1/2007 11:20:29 PM EDT
[#23]
Soon as I get a pic I'll post it. Rifle works fine though.
JFA |
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USA
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Posted: 1/2/2007 1:15:04 PM EDT
[#24]
Randall,
Here's a couple pics. Let me say first that I tried gas tube into the key to see if it fit okay and it did not, not even with some force. So I chucked it into the lathe and stoned with a very fine hone stone (about like crocus cloth) and that did it.   JFA |
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Posted: 1/2/2007 2:01:52 PM EDT
[#25]
That should have been the first clue that something was not right. |
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USA
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Posted: 1/2/2007 2:21:47 PM EDT
[#26]
It works and there's isn't an excessive amount of carbon in the receiver. So you think I should change it out?
JFA |
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Posted: 1/2/2007 2:23:41 PM EDT
[#27]
Damn...That was a good read, thank you Sir!
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TX, USA
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Posted: 4/5/2007 10:00:47 PM EDT
[#28]
+1 if you were my customer, I would advise you to get a new tube. |
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Posted: 4/8/2007 5:45:29 AM EDT
[#29]
Bumping to make notice that updates have been added to the original post with more detailed information about gas system dwell time.
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Posted: 4/8/2007 6:19:08 PM EDT
[#30]
good stuff. Thanks Randall.
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Posted: 4/8/2007 7:06:45 PM EDT
[#31]
From Randall's original post:
Upon reaching the gas key bolted to the top of the carrier, it turns down into the bolt carrier where it is given a nice place to expand. This is the area inside the bolt carrier where the bolt lives. Gas expanding here forces the bolt carrier back AND the bolt forward. Note that the bolt is also being forced BACK by the gas pressure expanding the cartridge case on the other side of the bolt. For a short moment in time, these forces are about equal. Many people do not realize this happens, or how very important it is. This pressure equalization relieves the force on the back of the bolts locking lugs and allows for easy unlocking of the bolt, and much reduced stress and wear. This relief of force does not happen in a piston/operating rod type system. BTW, the tail of the bolt IS a piston and the chamber of the bolt carrier IS a cylinder. The direct impingment system is not nearly the problem many claim it is, and it has some distinct advantages. |
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Posted: 4/8/2007 8:39:25 PM EDT
[#32]
sweet post Randall
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Posted: 4/14/2007 1:25:13 AM EDT
[#33]
Yes indeed it does, like straight-line thrust. |
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Posted: 4/14/2007 2:40:20 AM EDT
[#34]
and its so so freaking simple |
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NY, USA
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Posted: 4/16/2007 11:09:11 AM EDT
[#35]
Hey Randall,
Here's a neat little animation to go with your post: www.barnesengineering.com/AR15animation/index.htm. Justin |
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Posted: 5/2/2007 1:03:38 AM EDT
[#36]
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OK, USA
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Posted: 5/26/2007 5:00:28 PM EDT
[#37]
Very helpful it answered many of my new to the AR's operation questions. Thanks for taking your time to post it.
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Posted: 5/31/2007 6:49:02 AM EDT
[#38]
remember that "not enough gas" symptoms can also be caused by TOO MUCH BUFFER.
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Posted: 5/31/2007 11:02:58 AM EDT
[#39]
Absolutely! Everytime someone posts how their mid-length 14.5" gun short strokes with a 9mm buffer, I have to tell them to try a regular buffer. The response is always that "well, everyone on arfcom says to run an H2, H3 or 9mm buffer". What they don't realize is that you only can do that on 16" carbines, not on 14.5" mid-lengths... |
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Posted: 5/31/2007 2:54:28 PM EDT
[#40]
oh no, they will run on 14.5's......but only after there has been significant gas port erosion... in fact, a heavier buffer is a great way to deal with "too much gas" caused by gas port erosion on an old barrel. by the by, i know that YOU know all of this stuff....just for the kiddies at home. |
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Posted: 5/31/2007 3:09:00 PM EDT
[#41]
Mid-length gas ports don't erode nearly as much as carbine length gas ports. Rifle ports do it even less. |
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Posted: 5/31/2007 3:13:51 PM EDT
[#42]
shit.....pardon me.....i didnt notice MIDLENGTH in your previous post. |
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Posted: 6/1/2007 4:27:20 AM EDT
[#43]
hey Randall, do you have graphs like those on the first page for .308 as well?
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Posted: 8/24/2007 4:19:23 PM EDT
[Last Edit: CCW]
[#44]
The Barnes Engineering animated graphic is a great piece of work, and shows the AR15 action very well. However, the graphic shows the gas for the bolt operation dumping into the wrong side of the piston rings or bolt rings. This could be confusing to some because it does not agree with the text posted by Randall and others. The operating gas has to dump into the chamber between the bolt and the bolt carrier BEHIND the bolt rings, not in front of the bolt rings. (In this case "front" is toward the muzzle, "BEHIND" is toward the buffer.) |
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Posted: 9/19/2007 4:08:18 AM EDT
[#45]
I'm not sure I understand that chart. So which is better--- a 16 barrel with a carbine gas system or a 14.5 barrel with a mid-length gas system?
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USA
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Posted: 10/10/2007 2:18:00 AM EDT
[#46]
So I assume for a 16" Barrel, midlength gas system, & a wylde chamber the best buffer to use is either a straight normal carbine buffer or maybe just a normal H-buffer?
Thanks for this write up... VERY informative and helpful! |
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IN, USA
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Posted: 11/1/2007 9:32:03 PM EDT
[#47]
Great article - I use Ql, but I am racking my brain trying to back calculate the dwell time figures you came up with. I was converting the velocity from fps to inch per sec. I would divide the barrel lengh from chamber to gas port (and also to muzzle) - giving millisec, but I cannot duplicate your values.
Where am I going wrong? I am having troubles reloading for a 11.5" barrel AR. I am short stroking due to low pressure or lack of pressure in the gas tube. (I believe) Factory ammo works fine. I dupicated PMC valuies in QL by disassembly and measurement of components and determinging from an ex-pmc employee of the poweer they use. Their pressure is about 67K psi (QL data) - much higher than I care to reload to. Thanks |
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IN, USA
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Posted: 11/4/2007 9:03:36 PM EDT
[#48]
I figured it out. I could not delete my past thread.
Thanks Scott |
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MN, USA
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Posted: 3/27/2008 10:30:27 PM EDT
[Last Edit: oryx]
[#49]
this is an excellent post and absolutely tons of great info
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MI, USA
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Posted: 11/28/2008 5:15:41 PM EDT
[#50]
Hello all!
This site is amazing. I rarely need to post with so many questions already answered. God bless the search function! I am a little confused on the dwell time vs. max pressure. I am considering a 16" CMMG carbine with a rifle-length system. CMMG M10 Full Length MedCon Rifle (scroll down) After reading this topic, I am now unsure if that is a wise choice. The charts Randall was so kind to produce show: 16" barrel with rifle gas = ~18k PSI and .078 dwell time 14.5" barrel with carbine gas = ~25k PSI and .206 dwell time Can someone explain which would be worse and why? Shorter dwell time vs higher pressure? I thought I had it figured out until I started to compare gas systems and barrel lengths. If .2 is the optimum dwell time, does that negate the higher pressures the carbine would be submitted to? I know that there are other options, but these were my two main selections. Carbine and rifle gas are more commonly available, and I am set on a 16" or less barrel. Longevity and reliability are the key concerns for the rifle. Am I suffering from Acute Anal Black Rifle Disease? 
Thanks for any help! S/F, Lee |
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